Feature request: add `additionalFormParams` to `authConfigs`

In oauth2-authorize.js the additionalQueryStringParams property is used from the authConfigs object in order to add custom query string parameters to an authentication request.

Could we get a similar additionalFormParams property added that could then be passed to the authActions via method params or a property? That would allow actions.js to add custom form properties as it's building the form data string.

Reason:
We're trying to use Swagger-UI (via Swashbuckle.AspNetCore) to authenticate with Auth0. It has the right mix of params for the implicit grant type, but we're finding that client_credentials and password out-of-the-box body params passed aren't meeting Auth0's requirements. For example: when trying to use client_credentials, Auth0 requires audience in the body, but swagger-ui only passes grant_type and scope. Adding audience to the query string params doesn't cut it. This is where people are talking about getting the "Non-global clients are not allowed access to APIv1" error from Auth0.

If we had a similar way to add custom form/body params like we do query string params, it would make life easier.

Cross-post side note: ideally the Swashbuckle.AspNetCore library could then add another extension method similar to OAuthAdditionalQueryStringParams that would feed those values through in a similar way.

Auth0 Reference:

• client_credentials grant type requirements
• password grant type requirements

Start in src/core/oauth2-authorize.js, where additionalQueryStringParams is read, and trace how authConfigs reaches the auth actions in src/core/plugins/auth/actions.js. Check how the form data string is built and define the completed behavior as custom additionalFormParams being passed through and included in the authentication request body.