chore: Add Assignable Agents API (#4722)

app/controllers/api/v1/accounts/assignable_agents_controller.rb

@@ -0,0 +1,24 @@
+class Api::V1::Accounts::AssignableAgentsController < Api::V1::Accounts::BaseController
+  before_action :fetch_inboxes
+
+  def index
+    agent_ids = @inboxes.map do |inbox|
+      authorize inbox, :show?
+      member_ids = inbox.members.pluck(:user_id)
+      member_ids
+    end
+    agent_ids = agent_ids.inject(:&)
+    agents = Current.account.users.where(id: agent_ids)
+    @assignable_agents = (agents + Current.account.administrators).uniq
+  end
+
+  private
+
+  def fetch_inboxes
+    @inboxes = Current.account.inboxes.find(permitted_params[:inbox_ids])
+  end
+
+  def permitted_params
+    params.permit(inbox_ids: [])
+  end
+end