feat(web-api): add request interceptor and HTTP adapter config to WebClient [ISSUE-2073]

[mtjandra]

Summary

Resolves https://github.com/slackapi/node-slack-sdk/issues/2073

Features

• ability to configure one axios request interceptor
  • useful for clients who would like to manipulate outgoing requests to conform to their custom proxies
• ability to configure an axios adapter
  • useful for client who would like to used their existing http client
    • ex: client may have a dedicated http client configured to be used with their internal proxy

Contribution Pre-Requisites

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @mtjandra to sign the Salesforce Inc. Contributor License Agreement.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.23656% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.83%. Comparing base (2af216c) to head (5dabe0f). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2076 +/- ## ========================================== + Coverage 91.75% 91.83% +0.07% ========================================== Files 38 38 Lines 10093 10264 +171 Branches 631 646 +15 ========================================== + Hits 9261 9426 +165 - Misses 820 826 +6 Partials 12 12 ``` | [Flag](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | Coverage Δ | | |---|---|---| | [cli-hooks](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.23% <ø> (ø)` | | | [cli-test](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `95.69% <ø> (ø)` | | | [oauth](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `77.39% <ø> (ø)` | | | [socket-mode](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `58.22% <ø> (ø)` | | | [web-api](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.88% <96.23%> (-0.02%)` | :arrow_down: | | [webhook](https://app.codecov.io/gh/slackapi/node-slack-sdk/pull/2076/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=slackapi) | `96.65% <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=slackapi#carryforward-flags-in-the-pull-request-comment) to find out more.

[mtjandra]

Awesome work! Left a few minor documentation suggestions; once applied or rejected, I will merge this in.

Thank you so much for your work here and patience with me during review, it is greatly appreciated 🙇

Thank you for helping me with this and being responsive! I've addressed your comments and have rebased with main