fix: use correct content type for token request

[ewanharris]

Description

Moves the client to use application/x-www-form-urlencoded for the content type when making token requests per the spec.

In order to do this we needed to move from using StringContent to FormUrlEncodedContent and then the runtime will handle setting the content type header for us.

Also syncs some changes from the sdk-generator repo.

References

Part of openfga/sdk-generator#284
Generated from openfga/sdk-generator#308

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected