Create new configurator endpoint for custom event schemas

After the custom_event_schemas tables are created for new and existing projects as part of #9 (closed) and #10 (closed), respectively, an endpoint to save custom events should be added.

Endpoint requirements

The endpoint should accept a JSON payload which contains:

1. The project ID(s) which these custom events should be stored against (so we can get the correct project DB).
2. Any array of custom events which contains:
   • The custom events' definition:
     • Custom event name
     • Previous custom event name (if name is changed)
     • Custom event title (optional)
     • Custom event description (optional)
     • Custom event props
       • Prop key
       • Prop type (options: string, number, boolean, time)
       • Prop title (optional)
       • Prop description (optional)
   • The state of the custom event:
     • created
     • updated
     • deleted
   • If the custom event was updated, what the old custom event name was.
3. The endpoint should then apply the correct INSERT, ALTER, DELETE SQL functions based upon the state of the funnel.

Proposed input body

{
  "project_ids": ["gitlab_project_1", "gitlab_project_2"],
  "custom_events": [
    {
      "name": "Custom event 1",
      "title": "Overridden custom event title",
      "description": "Custom event description",
      "props": [
        {
          "key": "action",
          "type": "string",
          "title": "Overridden custom event property title",
          "description": "Custom event property description"
        }
      ],
      "state": "created"
    },
    {
      "name": "Custom event 2",
      "previous_name": "Custom event second",
      "props": [
        {
          "key": "action",
          "type": "string",
          "title": "Overridden custom event property title",
          "description": "Custom event property description"
        },
        {
          "key": "label",
          "type": "string"
        }
      ],
      "state": "updated"
    },
    {
      "name": "Custom event 3",
      "state": "deleted"
    }
  ]
}

This is based upon the assumption that the set-up would be the same as the current funnels system: https://gitlab.com/gitlab-org/analytics-section/analytics-configurator/-/issues/30+.

Implementation plan

In analytics-manager

• Add a new authorised route /custom-events-schemas, In this endpoint
  • Validate presence of name and props fields.
  • Loop over project_ids in the request, and for each project handle the custom event by
    • Creating a new record if the state is created
    • Deleting old record and creating a new one if the state is updated
    • Deleting a record if the state is deleted