Add endpoint for contacts autocomplete

What does this MR do and why?

It adds contacts as autocomplete source:

• adds a new route
• adds an endpoint to the controller + a new method to the AutocompleteService
• adds new finder for contacts

An existing feature flag is used here, this is not a user facing change. Using the endpoint should probably be under a new feature flag.

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:customer_relations)

2. Visit any group general settings http://127.0.0.1:3000/groups/flightjs/-/edit
3. Enable the custom relations in Permissions & group features (last item in the section)
4. Check if you have any existing contacts (http://127.0.0.1:3000/groups/flightjs/-/crm/contacts)
5. Visit the contacts autocomplete endpoint (project) http://127.0.0.1:3000/flightjs/Flight/-/autocomplete_sources/contacts
6. You should see a json with contacts (id-email pair)

You can play around and verify that eg.

• contacts are not returned if a user is not a member of a root group but is a member of a subgroup
• contacts of a group with the feature disabled are not returned
• 404 is returned when user does not have permissions to see the contacts (at least reporters can see the contacts)

Database

ContactsFinder queries

Query plans based on group https://gitlab.com/gitlab-org/protect/demos/sandbox (id: 14491403) in order to have some hierarchy to search

Getting group root ancestor

• existing method used: group = params[:group]&.root_ancestor
• query plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/8111/commands/28875

Checking permissions

• existing method used: can?(@current_user, :read_crm_contact, group)
• query plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/8111/commands/28876
• query plan against gitlab-org: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/8149/commands/29007

Getting contacts

• we don't have any data

group.contacts

SELECT "customer_relations_contacts".* FROM "customer_relations_contacts" 
  WHERE "customer_relations_contacts"."group_id" = 14491403

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #350133 (closed)