Expose project access token value with create API

What does this MR do?

#238991 (closed) introduced the ability to manage project access tokens via the API. But because we don't expose the token value, the create endpoint isn't useful since users can't access or use that value for anything. We should return the token value when using the create endpoint.

This MR exposes the project access token value upon creation so the create endpoint is useful. The API does not expose the value through GET because that is a security risk.

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

Related to #323088 (closed)