Implement form_post response mode

[Harm-r]

This pull request implements support for the form_post response mode, specifically:

• Support for the response_mode parameter as described in https://openid.net/specs/oauth-v2-multiple-response-types-1_0.html#ResponseModes
• Support for the form_post response mode as described in https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html
• Configuration options in the admin interface to enforce specific response modes per client via multiselect
• Unit and conformance tests for the form_post response mode

To achieve this, I've refactored the code to use an instance of the ResponseModeInterface for returning responses, instead of a hardcoded queryDelimiter parameter.

This work has been done during the 11th cycle of the T&I Incubator at GÉANT.

[codecov]

Codecov Report

❌ Patch coverage is 41.28440% with 128 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.88%. Comparing base (6342151) to head (5460a76).

Files with missing lines	Patch %	Lines
src/Server/Exceptions/OidcServerException.php	0.00%	47 Missing ⚠️
src/Server/Grants/AuthCodeGrant.php	0.00%	15 Missing ⚠️
src/Services/Container.php	0.00%	10 Missing ⚠️
src/Forms/ClientForm.php	71.42%	8 Missing ⚠️
src/Entities/ClientEntity.php	0.00%	7 Missing ⚠️
src/Factories/RequestRulesManagerFactory.php	0.00%	7 Missing ⚠️
src/Server/AuthorizationServer.php	0.00%	5 Missing ⚠️
src/Server/Grants/ImplicitGrant.php	50.00%	5 Missing ⚠️
src/Server/ResponseModes/FragmentResponseMode.php	0.00%	5 Missing ⚠️
src/Server/ResponseModes/QueryResponseMode.php	0.00%	5 Missing ⚠️
... and 7 more

Additional details and impacted files

@@                 Coverage Diff                 @@
##             wip-version-7     #337      +/-   ##
===================================================
- Coverage            46.88%   46.88%   -0.01%     
- Complexity            2041     2074      +33     
===================================================
  Files                  166      171       +5     
  Lines                 8662     8816     +154     
===================================================
+ Hits                  4061     4133      +72     
- Misses                4601     4683      +82     

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