feat(enterprise): add voice conference API (#13064)

The backend APIs for the voice call channel 
ref: #11602

enterprise/app/controllers/api/v1/accounts/conference_controller.rb

@@ -0,0 +1,58 @@
+class Api::V1::Accounts::ConferenceController < Api::V1::Accounts::BaseController
+  before_action :set_voice_inbox_for_conference
+
+  def token
+    render json: Voice::Provider::Twilio::TokenService.new(
+      inbox: @voice_inbox,
+      user: Current.user,
+      account: Current.account
+    ).generate
+  end
+
+  def create
+    conversation = fetch_conversation_by_display_id
+    ensure_call_sid!(conversation)
+
+    conference_service = Voice::Provider::Twilio::ConferenceService.new(conversation: conversation)
+    conference_sid = conference_service.ensure_conference_sid
+    conference_service.mark_agent_joined(user: current_user)
+
+    render json: {
+      status: 'success',
+      id: conversation.display_id,
+      conference_sid: conference_sid,
+      using_webrtc: true
+    }
+  end
+
+  def destroy
+    conversation = fetch_conversation_by_display_id
+    Voice::Provider::Twilio::ConferenceService.new(conversation: conversation).end_conference
+    render json: { status: 'success', id: conversation.display_id }
+  end
+
+  private
+
+  def ensure_call_sid!(conversation)
+    return conversation.identifier if conversation.identifier.present?
+
+    incoming_sid = params.require(:call_sid)
+
+    conversation.update!(identifier: incoming_sid)
+    incoming_sid
+  end
+
+  def set_voice_inbox_for_conference
+    @voice_inbox = Current.account.inboxes.find(params[:inbox_id])
+    authorize @voice_inbox, :show?
+  end
+
+  def fetch_conversation_by_display_id
+    cid = params[:conversation_id]
+    raise ActiveRecord::RecordNotFound, 'conversation_id required' if cid.blank?
+
+    conversation = @voice_inbox.conversations.find_by!(display_id: cid)
+    authorize conversation, :show?
+    conversation
+  end
+end