Allow for extending OAuth2AuthorizedClient with additional parameters in the code grant flow

Expected Behavior

On successful authorization, the stored authorized client includes any additional parameters provided by the auth server during the code grant flow.

Current Behavior

OAuth2AuthorizationCodeGrantFilter currently creates a OAuth2AuthorizedClient directly and then saves it via the OAuth2AuthorizedClientRepository. It doesn't provide any means to include additional parameters from the authorization step. The OAuth2AuthorizationCodeAuthenticationToken has additionalParameters already.

As an example of how it could be modified to pass the parameters:

1. Extend OAuth2AuthorizedClient to include and additionalParameters map e.g.: private Map<String, Object> additionalParameters = new HashMap<>();

2. Modify OAuth2AuthorizationCodeGrantFilter to copy the additionalParameters from the OAuth2AuthorizationCodeAuthenticationToken to the OAuth2AuthorizedClient

Context

I'm using an API that includes a couple of extra parameters in the code grant flow and the client needs to include these parameters in headers in order to use the API. I can't currently see a way to get those parameters stored with the authorized client without re-writing the filter.

Start by reading OAuth2AuthorizedClient, OAuth2AuthorizationCodeGrantFilter, and OAuth2AuthorizationCodeAuthenticationToken to trace how authorization results are stored. Verify how OAuth2AuthorizedClientRepository receives the client and define completion as preserving the authorization step's additional parameters for later API use; the payload names no test file or command.