Expose update project service JSON endpoint

What does this MR do?

This MR enables the support of updating project services via JSON.

Utilize json_fields to expose fields via Service#as_json(only: json_fields).

Needed for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15733

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/13203

This MR also cleans up the project services specs a bit. To see a diff without whitespace changes see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32759/diffs?w=1

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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