Allow cross-origin requests on /oauth/token

What does this MR do?

Fixes #300077 (closed)

It allows CORS requests when recovering the token in an OAuth flow. This is required for the PKCE flow to work from a browser, which is the intended use-case for that flow as well as mobile applications.

CORS is enabled for POST requests only, cookies and other custom headers aren't allowed.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team