Add API for admin users to create other user's PATs

What does this MR do?

Adds the following API for admins to create PAT for other users:

[POST] /users/:user_id/personal_access_tokens

Example payload:

{
    "name": "test token",
    "scopes": ["api"],
    "expires_at": "2020-12-01"
}

This is not a new ability for the admins. They can already do this today via the UI by impersonating the user.

We added additional logging to ensure auditing of this feature by adding application and audit logs to PAT creation (and revocation) service in this MR: !45976 (merged)

The new API is behind a feature flag pat_creation_api_for_admin defaulted to false.

Related to #17176 (closed)

Screenshots

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 #17176 (closed)