feat(otlp-exporter-base): internally accept a http header provider function only

[pichlermarc]

Which problem is this PR solving?

Changes internal code to always expect a header provider function, instead of a direct object. This enables us to implement #2903 later.

This does not fully implement #2903 as I want to migrate the current way of constructing an OTLP exporter to a factory function. That factory function will have a slightly different options to the existing OTLP exporters. Goal of introducing the factory function with a different interface is to:

• reduce the public API (hide private properties and avoid that users can inherit from exporters - offering them a composition-based approach instead)
• move to a single way of setting options (like keepAlive, or inheaders in this case)
  • a single case to handle makes the underlying code simpler and makes setup code more consistent across users, which should aid in hunting down bugs

The changes in this PR are currently non-breaking as it's not part of the public API in any released version. So it's just internally breaking if we merge it before we release the changes from #5031.

Enables #2903

Short description of the changes

Type of change

• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• [x] existing unit tests
• [x] adapted unit tests

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.56%. Comparing base (91b9abd) to head (909666f). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5179 +/- ## ========================================== - Coverage 94.57% 94.56% -0.01% ========================================== Files 314 314 Lines 7956 7966 +10 Branches 1600 1601 +1 ========================================== + Hits 7524 7533 +9 - Misses 432 433 +1 ``` | [Files with missing lines](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | Coverage Δ | | |---|---|---| | [.../configuration/convert-legacy-node-http-options.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Fconfiguration%2Fconvert-legacy-node-http-options.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvY29uZmlndXJhdGlvbi9jb252ZXJ0LWxlZ2FjeS1ub2RlLWh0dHAtb3B0aW9ucy50cw==) | `100.00% <100.00%> (ø)` | | | [...-base/src/configuration/otlp-http-configuration.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Fconfiguration%2Fotlp-http-configuration.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvY29uZmlndXJhdGlvbi9vdGxwLWh0dHAtY29uZmlndXJhdGlvbi50cw==) | `100.00% <100.00%> (ø)` | | | [...e/src/configuration/otlp-http-env-configuration.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Fconfiguration%2Fotlp-http-env-configuration.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvY29uZmlndXJhdGlvbi9vdGxwLWh0dHAtZW52LWNvbmZpZ3VyYXRpb24udHM=) | `95.83% <100.00%> (+0.08%)` | :arrow_up: | | [...ter-base/src/configuration/shared-configuration.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Fconfiguration%2Fshared-configuration.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvY29uZmlndXJhdGlvbi9zaGFyZWQtY29uZmlndXJhdGlvbi50cw==) | `100.00% <100.00%> (ø)` | | | [...xporter-base/src/transport/http-transport-utils.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Ftransport%2Fhttp-transport-utils.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvdHJhbnNwb3J0L2h0dHAtdHJhbnNwb3J0LXV0aWxzLnRz) | `100.00% <ø> (ø)` | | | [...perimental/packages/otlp-exporter-base/src/util.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179?src=pr&el=tree&filepath=experimental%2Fpackages%2Fotlp-exporter-base%2Fsrc%2Futil.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-ZXhwZXJpbWVudGFsL3BhY2thZ2VzL290bHAtZXhwb3J0ZXItYmFzZS9zcmMvdXRpbC50cw==) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5179/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry)

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