Add endpoint to regenerate feature flags client token

What does this MR do?

This MR contains the backend changes needed for https://gitlab.com/gitlab-org/gitlab-ee/issues/11417. It adds new POST /*namespace_id/:project_id/-/feature_flags_client/reset_token.json endpoint to be used from the frontend to reset the token for the project's feature flags client. Response will be 200 OK and will contain the newly generated token value. User needs to have admin_feature_flag permission.

# Request
#  POST /*namespace_id/:project_id/-/feature_flags_client/reset_token.json 

# Response
# 200 OK
{
  "token" => "new-token-value"
}

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

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/11417.