Abstract out tokens from being tied tightly to access

[bradjones1]

Some OAuth2 clients also support OpenID Connect (OIDC.) Also, some native app integrations (e.g., Google) directly return OIDC ID tokens to the native app, which then can be used by a backend for SSO. You get the idea.

There are some other issues/PRs open for OIDC functionality, e.g. #899, and those are good starts for sure. I think what is missing from at least that PR, and more generally, is the ability to represent tokens in a more abstract way. E.g., an OIDC ID token represents a resource owner, but does not contain an access token. This can also lead to some confusion like in #976.

See also thephpleague/oauth2-google#119 for the Google provider, which is an example of how these libraries can be augmented with ID token support to better serve as a backend for all types of SSO implementations, not just browser-based OAuth2 flow.

[bradjones1]

I realize the code style is a bit messed up here but am jamming on a project and figure if maintainers are into this, it's easy enough to clean up. Would perhaps be nice to ship some composer commands to do CS linting and fixing locally.

[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.24%. Comparing base (2455200) to head (9cc147c).
⚠️ Report is 58 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Token/IdToken.php	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              master     #981      +/-   ##
=============================================
- Coverage     100.00%   99.24%   -0.76%     
- Complexity       191      196       +5     
=============================================
  Files             20       22       +2     
  Lines            516      527      +11     
=============================================
+ Hits             516      523       +7     
- Misses             0        4       +4     

Files with missing lines	Coverage Δ
src/Token/AbstractToken.php	100.00% <100.00%> (ø)
src/Token/AccessToken.php	100.00% <100.00%> (ø)
src/Token/IdToken.php	0.00% <0.00%> (ø)

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