fix: Omit non-standard, empty fields in `RefreshTokenRequest` when performing a token refresh

[ymarcus93]

The OIDC spec's definition of a refresh request does not include client_assertion or client_assertion_type as valid parameters for the refresh request. See request format here: https://openid.net/specs/openid-connect-core-1_0.html#RefreshingAccessToken. The document only displays client_id, client_secret, grant_type, refresh_token, and scope as acceptable parameters.

Therefore, I propose we add the omitempty tags to the ClientAssertion and ClientAssertionType fields in RefreshTokenRequest, so that the token refresh functionality provided by rp.RefreshTokens can work with identity providers that may have additional logic or different expectations when these fields are included in the refresh token request.

For example, when attempting to construct an OIDC client via rp.RelyingParty against an Okta identity provider, I ran into issues when performing refresh with rp.RefreshTokens. The Okta identity provider returned http status not ok: 400 Bad Request {"error":"invalid_request","error_description":"The client_assertion_type is invalid."} as an error. I assume I'm receiving this error because I'm calling the RefreshTokens func with clientAssertion="" and clientAssertionType=""; the addition of the omitempty tags resolves this issue and hopefully future proofs this library against other identity providers that have the same behavior.

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[livio-a]

hey @ymarcus93 thanks for the fix and totally agree on that. Since scope and depending on the authentication also client_id and client_secret are optional, I'd add the omitempty tag there as well.

[ymarcus93]

@livio-a Rebased and pushed b03e835 with omitempty on scope, client_id, and client_secret

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 62.04%. Comparing base (0992c5f) to head (b03e835). Report is 56 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #599 +/- ## ========================================== + Coverage 60.06% 62.04% +1.98% ========================================== Files 80 81 +1 Lines 6998 6192 -806 ========================================== - Hits 4203 3842 -361 + Misses 2498 2041 -457 - Partials 297 309 +12 ```

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:tada: This PR is included in version 3.23.2 :tada:

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