Split codecov upload to separate github actions workflow

[AndreKurait]

Description

Split codecov upload to separate github actions workflow.

This enables making the CI workflow trigger on pull_request as it will not need secret access making modifications much easier.

• Category: Enhancement
• Why these changes are required? Changes to the CI workflow are common to unblock the pipeline and it can impede PRs to make it pull_request_target
• What is the old behavior before changes and new behavior after changes?

Issues Resolved

[List any issues this PR will resolve]

Is this a backport? If so, please add backport PR # and/or commits #

Testing

Lots of testing on my fork with push, pr, and pr from a fork. E.g.: https://github.com/AndreKurait/opensearch-migrations/pull/7

Check List

• [x] New functionality includes testing
  • [x] All tests pass, including unit test, integration test and doctest
• [x] New functionality has been documented
• [x] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov[bot]]

Codecov Report

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

Project coverage is 80.72%. Comparing base (5514bc7) to head (1494a67). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1139 +/- ## ========================================= Coverage 80.72% 80.72% Complexity 2947 2947 ========================================= Files 399 399 Lines 14965 14965 Branches 1017 1017 ========================================= Hits 12080 12080 Misses 2274 2274 Partials 611 611 ``` | [Flag](https://app.codecov.io/gh/opensearch-project/opensearch-migrations/pull/1139/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | Coverage Δ | | |---|---|---| | [gradle-test](https://app.codecov.io/gh/opensearch-project/opensearch-migrations/pull/1139/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `78.74% <ø> (ø)` | | | [python-test](https://app.codecov.io/gh/opensearch-project/opensearch-migrations/pull/1139/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `89.95% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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[peternied]

@opensearch-project/admin Can you force merge this change, we accidentally broke our required CI checks and this will resolve that issue.