Add configuration of access_level for runners on registration via API

What does this MR do?

Allow users to set the access_level of a runner upon registration via API. Currently you are only able to do so by the UI at the runner admin panel or via the API for updating runners.

Relevant docs and tests are updated to reflect the change above.

See gitlab-runner#3186 (closed)

See gitlab-runner!1323 (merged) for the gitlab-runner side of the code to allow configuring access_level at registration

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance 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

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