Add token request parameters for authorization code flow

[coberlin]

The access token request needs the code if we're using code flow. For authorization code flow, grant_type and redirect_uri are also required. See https://docs.duendesoftware.com/identityserver/v7/reference/endpoints/token/

To run tests, pin the minitest version per a conflict with mocha, as noted here: freerange/mocha#614
But do not leave pinned as the gem fails to install under some rubies that previously succeeded.

For example:
net-imap-0.5.0 requires ruby version >= 3.1.0, which is incompatible with the current version, ruby 2.6.8p0 (jruby 9.3.7.0)

[dominikklein]

Would be nice to have this merged with one extension:

token_request_params[:client_id] = client_options.identifier

Then for example also auth0 would be working with this gem. At least in my tests.

[dominikklein]

@coberlin Maybe you can add this to your changes here.

@stanhu @bufferoverflow Any chance that this can be merged?

[bufferoverflow]

@dominikklein I miss a test and I think might be worth todo a code suggestion regarding your comment.

[dominikklein]

Then I will try to add a new PR, with a test :-)

[coberlin]

@dominikklein Your PR #199 looks good to me! I'll close my PR. Thank you