feat: Telegram Channel (#2901)

- Ability to configure telegram bots as a channel in chatwoot
- Receive a message sent to the telegram bot in chatwoot
- Ability to reply to telegram users from chatwoot
- Receive attachment messages in chatwoot

fixes: #1843

db/migrate/20210828124043_add_telegram_channel.rb

@@ -0,0 +1,10 @@
+class AddTelegramChannel < ActiveRecord::Migration[6.1]
+  def change
+    create_table :channel_telegram do |t|
+      t.string :bot_name
+      t.integer :account_id, null: false
+      t.string :bot_token, null: false, index: { unique: true }
+      t.timestamps
+    end
+  end
+end