Add support for client credentials grant

[nick-vanpraet]

PR to replace #257 as that one seems abandoned.

Adds support for the client_credentials grant type added in M4 mautic/mautic#9837

I kept as much of the logic that was used in the Oauth Auth class in as I could (the debugging, the weird query parameter access token thing that I'm pretty sure is a security concern, etc).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 72 lines in your changes missing coverage. Please review.

Project coverage is 48.09%. Comparing base (7478f55) to head (c445d71).
Report is 111 commits behind head on main.

Files	Patch %	Lines
lib/Auth/TwoLeggedOAuth2.php	0.00%	72 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #269      +/-   ##
============================================
- Coverage     51.45%   48.09%   -3.37%     
- Complexity      406      434      +28     
============================================
  Files            30       31       +1     
  Lines          1028     1100      +72     
============================================
  Hits            529      529              
- Misses          499      571      +72     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[kuzmany]

Don't understand create duplicate PR of #257

[RCheesley]

@kuzmany guessing because there has been no response to feedback since September 2021!

[kuzmany]

Ok. But still would be good mention what is different between my version and this version.
We use that PR on production, works as expected and still don't see reason create new PR

[nick-vanpraet]

Sure thing.

• Requires baseUrl to be given
• Implements accessTokenUpdated (for those who want it, gets set to true in requestAccessToken if nothing failed)
• Implements getAccessTokenData method to return token, expires and token type
• Checks the access token's expiry and presence in isAuthorized/validateAccessToken so code can re-use access tokens more easily
• Implements getQueryParameters, straight copy from Oauth class. I imagine it's there to fix a bug, based on the comment above it.
• Supports $this->_debug variable the same way Oauth class does

[RCheesley]

It would be great to get a decision made on this @escopecz @kuzmany and merge it if we want to accept it in favour of the outdated PR based on the feedback provided above.