support response_mode=form_post in upstream OIDC IDPs

[cfryanr]

Addresses #2238.

This PR makes it possible to use response_mode=form_post with an OIDCIdentityProvider.

For example:

apiVersion: idp.supervisor.pinniped.dev/v1alpha1
kind: OIDCIdentityProvider
metadata:
  name: my-oidc-idp
  namespace: pinniped-supervisor
spec:
  issuer: https://my-oidc-provider.example.com/path
  authorizationConfig:
    additionalAuthorizeParameters:
      # This is the part that could not work before this PR.
      - name: response_mode
        value: form_post
      # End new part.
  claims:
    username: email
    groups: groups
  client:
    secretName: oidc-client-creds

This is typically not needed. However, see #2238 for a case where it might be needed when using certain versions of ADFS.

I manually tested that this works on my Mac laptop using Okta as the IDP and using the latest Safari, Chrome, Firefox, and Edge browsers (all for MacOS).

Release note:

The Pinniped Supervisor now supports using `response_mode=form_post` with an OIDCIdentityProvider.
Some versions of ADFS might require this in order for Pinniped to receive certain claims in the
ADFS-issued ID token.

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[codecov]

Codecov Report

Attention: Patch coverage is 83.05085% with 10 lines in your changes missing coverage. Please review.

Project coverage is 24.99%. Comparing base (308c76c) to head (749633e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ationdomain/endpoints/callback/callback_handler.go	82.75%	4 Missing and 1 partial ⚠️
internal/testutil/log_lines.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2254      +/-   ##
==========================================
+ Coverage   24.95%   24.99%   +0.04%     
==========================================
  Files         379      379              
  Lines       62172    62222      +50     
==========================================
+ Hits        15515    15555      +40     
- Misses      46322    46330       +8     
- Partials      335      337       +2     

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