feat: New APIs for search (#6564)

- Adding new API endpoints for search
- Migrations to add appropriate indexes

app/jobs/migration/add_search_indexes_job.rb

@@ -0,0 +1,17 @@
+# Delete migration and spec after 2 consecutive releases.
+class Migration::AddSearchIndexesJob < ApplicationJob
+  queue_as :scheduled_jobs
+
+  def perform
+    ActiveRecord::Migration[6.1].add_index(:messages, [:account_id, :inbox_id], algorithm: :concurrently)
+    ActiveRecord::Migration[6.1].add_index(:messages, :content, using: 'gin', opclass: :gin_trgm_ops, algorithm: :concurrently)
+    ActiveRecord::Migration[6.1].add_index(
+      :contacts,
+      [:name, :email, :phone_number, :identifier],
+      using: 'gin',
+      opclass: :gin_trgm_ops,
+      name: 'index_contacts_on_name_email_phone_number_identifier',
+      algorithm: :concurrently
+    )
+  end
+end