Expose projects with integration overrides

What does this MR do?

This MR exposes a JSON endpoint of projects that are using different integration settings other than what has been set on the instance admin-level. A frontend MR will later consume the data #218252 (closed).

Walking through the QA steps should help to explain the feature 🙂.

QA

1. Visit your instance admin's integration settings (/admin/application_settings/integrations), and configure the Campfire integration (we use Campfire in this example because it's a simple integration; it requires a token to be provided - just enter anything).
2. Visit a project's Campfire integration settings (/<project-full-path>/-/services/campfire/edit) and change the drop-down to Use custom settings. Save the integration (you don't need to change anything else in the form).
3. Visit the JSON endpoint for Campfire overrides (/admin/application_settings/integrations/campfire/overrides.json) and the project should appear in the list.
4. Edit the project's Campfire integration again, this time change the drop-down to Use default settings. Save the integration.
5. Visit the JSON endpoint for Campfire overrides and the project should not appear.

Example JSON output

[
  {
    "avatar_url": null,
    "full_name": "Administrator / test",
    "name": "test",
    "full_path": "/root/test"
  }
]

Example queries

We're using ConfluenceService here as the example integration type.

The queries produced by changes in Admin::IntegrationsController#overrides are:

The paginator count query (link to #database-lab result):

SELECT
   COUNT(*) 
FROM
   (
      SELECT
         1 AS one 
      FROM
         "projects" 
         INNER JOIN
            "integrations" 
            ON "integrations"."project_id" = "projects"."id" 
      WHERE
         "integrations"."type" = 'ConfluenceService' 
         AND "integrations"."active" = TRUE 
         AND "integrations"."inherit_from_id" IS NULL LIMIT 10001
   )
   subquery_for_count

The results query (link to #database-lab result)::

SELECT
   "projects".* 
FROM
   "projects" 
   INNER JOIN
      "integrations" 
      ON "integrations"."project_id" = "projects"."id" 
WHERE
   "integrations"."type" = 'ConfluenceService' 
   AND "integrations"."active" = TRUE 
   AND "integrations"."inherit_from_id" IS NULL 
ORDER BY
   "projects"."id" ASC LIMIT 20 OFFSET 0

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #218252 (closed)