Feat: Add extra params support to TokenParams

[rihardsgravis]

Some providers expect to send extra parameters (e.g. HubSpot expects the client_id to be present) to the token exchange endpoint, similar to the auhtorization URI.

[paul-hicks-nz]

Isn't the client_id added automatically in the request() method? If you set your authenticationMethod to client_secret_post, the client_id (and client_secret, if provided) are added to the request body.

[evert]

I'm closing this. If it comes up more often i'll reconsider!