Add issues set crm contacts service and graphql mutation

What does this MR do and why?

See #2256 for an overview on the Customer Relations / CRM feature and work completed so far.

This MR inroduces a new graphql mutation for setting/adding/removing customer relations contacts to/from issues (and the required service to support it)

Screenshots or screen recordings

How to set up and validate locally

From rails console:
1. Enable feature flag:
Feature.enable(:customer_relations)
From GraphiQL:
2. Pick a an issue (I used http://gdk.test:3000/flightjs/Flight/-/issues/30)
3. Create a few contacts for the group the issue belongs to (and take note of their ids):
mutation {
  customerRelationsContactCreate(input:
    {
      groupId: "gid://gitlab/Group/26"
      firstName: "Lee"
      lastName: "Tickett"
    }) {
    contact {
      id
      firstName
      lastName
    }
    errors
  }
}
4. Try and APPEND/REMOVE/REPLACE the contacts assigned to the issue:
mutation {
  issueSetCrmContacts(input:
    {
      projectPath: "flightjs/Flight"
      iid: "30"
      operationMode: APPEND
      crmContactIds: [
        "gid://gitlab/CustomerRelations::Contact/14",
        "gid://gitlab/CustomerRelations::Contact/12"
      ]
    }) {
    issue {
     id
      customerRelationsContacts {
        nodes {
          id
          firstName
          lastName
        }
      }
    }
    errors
  }
}
--
mutation {
  issueSetCrmContacts(input:
    {
      projectPath: "flightjs/Flight"
      iid: "30"
      operationMode: REPLACE
      crmContactIds: [
        "gid://gitlab/CustomerRelations::Contact/11",
        "gid://gitlab/CustomerRelations::Contact/12"
      ]
    }) {
    issue {
     id
      customerRelationsContacts {
        nodes {
          id
          firstName
          lastName
        }
      }
    }
    errors
  }
}
--
mutation {
  issueSetCrmContacts(input:
    {
      projectPath: "flightjs/Flight"
      iid: "30"
      operationMode: REMOVE
      crmContactIds: [
        "gid://gitlab/CustomerRelations::Contact/12"
      ]
    }) {
    issue {
     id
      customerRelationsContacts {
        nodes {
          id
          firstName
          lastName
        }
      }
    }
    errors
  }
}

MR acceptance checklist

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