feat: enable user to disable check for token exchange response body

[PixelWeaver]

Hi there, great lib, thanks for your work on this! I've been using a fork of it for a bit it's been working really well!

Some APIs (e.g. Officient) do not respond to the token exchange request with one of the recognized Content-Type headers, causing check_response_body to return false.

Adding the disable_check_response_body function to the builder of CodeTokenRequest would offer a trifle more flexibility to users of this library without altering the default behavior.

[ramosbugs]

What Content-Type header value is returned? The spec is pretty clear that the response body needs to be JSON:

The parameters are included in the entity-body of the HTTP response
using the "application/json" media type as defined by [RFC4627]

This library treats missing Content-Type headers as if they're JSON, so if check_response_body() is returning an error it means that either the Content-Type response header is explicitly set to something other than application/json, or the response body is missing entirely. Neither of those cases are compliant with the spec, so I don't think it makes sense to add explicit support to this library.

My suggestion, as for most other deviations from the spec, is to define a custom HTTP client as a shim (passed to request()/request_json()) that converts the non-compliant response into a compliant one before returning it to this library. For example, that could mean setting the Content-Type header to application/json in the HTTP response.