Return slug in /api/v4/projects/ID/services

What does this MR do?

It adds a new field slug to the response of /api/v4/projects/:id/services. The field contains the parameterized name of a service that can be user to perform update, show or delete on a single service.

closes #195795 (closed)

The response would look like this:

[
  {
    "id": 75,
    "slug": "jenkins",
    "title": "Jenkins CI",
    "created_at": "2019-11-20T11:20:25.297Z",
    "updated_at": "2019-11-20T12:24:37.498Z",
    "active": true,
    "commit_events": true,
    "push_events": true,
    "issues_events": true,
    "confidential_issues_events": true,
    "merge_requests_events": true,
    "tag_push_events": false,
    "note_events": true,
    "confidential_note_events": true,
    "pipeline_events": true,
    "wiki_page_events": true,
    "job_events": true
  }
]

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

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