Wrong JSON field name for Code in TokenRequest

[daic0r]

Bug report

Describe the bug

The field Code in types.TokenRequest has a wrong name in the struct field tag.
Therefore, code exchange for grant type "pkce" does not work and returns an error:

{"code":400,"error_code":"validation_failed","msg":"invalid request: both auth code and code verifier should be non-empty"}

The name in the field tag is currently code, but ought to be auth_code instead.

To Reproduce

1. Initiate a PKCE flow via the /auth/v1/authorize endpoint and save the returned code verifier in a cookie

2. Receive callback and parse code

3. Create a TokenRequest with the grant type "pkce" and set Code to the parsed code and the verifier to the one saved in step 1:
   

4. Call Token and pass the TokenRequest object from step 3

5. You will receive the error mentioned above

Expected behavior

No error, access token should be received correctly.

[kwoodhouse93]

Addressed by #6