feat: Support optional headers for OAuth request

[s7clarke10]

Adding support for adding a header to an OAuth request for tokens. In a few instances an API has a requirement to have headers added to the request for an OAuth Token. An example of the type of header is a subscription key.

This extends the current OAuth Authenticator to accept a new optional parameter oauth_headers.

The PR also explicitly converts the returned expires_in value to int rather than assuming it is an integer. I had an example where it was returned as an string from this API.

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📚 Documentation preview 📚: https://meltano-sdk--1815.org.readthedocs.build/en/1815/

[edgarrmondragon]

Thanks @s7clarke10! A few suggestions to simplify things a bit, let me know what you think :)

[codecov]

Codecov Report

Merging #1815 (067a7e0) into main (b323d45) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 067a7e0 differs from pull request most recent head 260d141. Consider uploading reports for the commit 260d141 to get more accurate results

@@           Coverage Diff           @@
##             main    #1815   +/-   ##
=======================================
  Coverage   86.46%   86.47%           
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  Files          59       59           
  Lines        4996     4998    +2     
  Branches      816      816           
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+ Hits         4320     4322    +2     
  Misses        479      479           
  Partials      197      197           

Impacted Files	Coverage Δ
singer_sdk/authenticators.py	64.02% <100.00%> (+0.38%)	⬆️

... and 1 file with indirect coverage changes

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[s7clarke10]

This PR has been tested on my enhancements to tap-rest-api-msdk. This is the branch that I have used for my testing https://github.com/s7clarke10/tap-rest-api-msdk/commits/feature/sdk_supported_oauth_headers .

Thanks for your support for this change and great suggestions to resolve the failed tests. I believe all the comments can be resolved now.

[s7clarke10]

fix: #1822

[edgarrmondragon]

Thanks @s7clarke10!