FEAT 222 Add client credential flow

[Andy-Grigg]

Closes #222

This PR includes a very rough initial implementation of the client credential flow from requests-auth:

• Renames the existing with_oauth() method to with_oauth_pkce()
• Introduces a new method with_oauth_client_credentials()

I suspect there's a lot of duplication here, and that this isn't necessarily the most user-friendly way of doing it. Adding another level of builder also doesn't seem sensible though.

PR is based on both #221 and #139 , hence the large number of commits.

Tested against a real service implementing OIDC client credential flow and confirmed working.

[codecov]

Codecov Report

Merging #223 (116847c) into main (fec10be) will increase coverage by 1.56%.
The diff coverage is 81.53%.

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   91.88%   93.44%   +1.56%     
==========================================
  Files           8        8              
  Lines         826      809      -17     
==========================================
- Hits          759      756       -3     
+ Misses         67       53      -14     

Impacted Files	Coverage Δ
src/ansys/openapi/common/_contrib/granta.py	100.00% <ø> (ø)
src/ansys/openapi/common/_util.py	99.39% <ø> (-0.05%)	⬇️
src/ansys/openapi/common/_api_client.py	97.17% <75.00%> (-0.75%)	⬇️
src/ansys/openapi/common/_oidc.py	85.60% <79.31%> (+11.10%)	⬆️
src/ansys/openapi/common/_session.py	86.45% <84.61%> (-1.12%)	⬇️
src/ansys/openapi/common/__init__.py	100.00% <100.00%> (ø)

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[Andy-Grigg]

Closing this to try and clean up diff.