feat: Introduce the crm_v2 feature flag for CRM changes (#12014)

Introduce crm_v2 feature flag for our upcoming optimisations for CRM

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

db/migrate/20250722152516_add_index_on_contact_type_and_account_id_to_contacts.rb

@@ -0,0 +1,7 @@
+class AddIndexOnContactTypeAndAccountIdToContacts < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :contacts, [:account_id, :contact_type], name: 'index_contacts_on_account_id_and_contact_type', algorithm: :concurrently
+  end
+end