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

app/models/channel/telegram.rb

@@ -0,0 +1,89 @@
+# == Schema Information
+#
+# Table name: channel_telegram
+#
+#  id         :bigint           not null, primary key
+#  bot_name   :string
+#  bot_token  :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#  account_id :integer          not null
+#
+# Indexes
+#
+#  index_channel_telegram_on_bot_token  (bot_token) UNIQUE
+#
+
+class Channel::Telegram < ApplicationRecord
+  self.table_name = 'channel_telegram'
+  EDITABLE_ATTRS = [:bot_token].freeze
+
+  has_one :inbox, as: :channel, dependent: :destroy
+  belongs_to :account
+
+  before_validation :ensure_valid_bot_token, on: :create
+  validates :account_id, presence: true
+  validates :bot_token, presence: true, uniqueness: true
+  before_save :setup_telegram_webhook
+
+  def name
+    'Telegram'
+  end
+
+  def has_24_hour_messaging_window?
+    false
+  end
+
+  def telegram_api_url
+    "https://api.telegram.org/bot#{bot_token}"
+  end
+
+  def send_message_on_telegram(message, chat_id)
+    response = HTTParty.post("#{telegram_api_url}/sendMessage",
+                             body: {
+                               chat_id: chat_id,
+                               text: message
+                             })
+
+    response.parsed_response['result']['message_id'] if response.success?
+  end
+
+  def get_telegram_profile_image(user_id)
+    # get profile image from telegram
+    response = HTTParty.get("#{telegram_api_url}/getUserProfilePhotos", query: { user_id: user_id })
+    return nil unless response.success?
+
+    photos = response.parsed_response['result']['photos']
+    return if photos.blank?
+
+    get_telegram_file_path(photos.first.last['file_id'])
+  end
+
+  def get_telegram_file_path(file_id)
+    response = HTTParty.get("#{telegram_api_url}/getFile", query: { file_id: file_id })
+    return nil unless response.success?
+
+    "https://api.telegram.org/file/bot#{bot_token}/#{response.parsed_response['result']['file_path']}"
+  end
+
+  private
+
+  def ensure_valid_bot_token
+    response = HTTParty.get("#{telegram_api_url}/getMe")
+    unless response.success?
+      errors.add(:bot_token, 'invalid token')
+      return
+    end
+
+    self.bot_name = response.parsed_response['result']['username']
+  end
+
+  def setup_telegram_webhook
+    HTTParty.post("#{telegram_api_url}/deleteWebhook")
+    response = HTTParty.post("#{telegram_api_url}/setWebhook",
+                             body: {
+                               url: "#{ENV['FRONTEND_URL']}/webhooks/telegram/#{bot_token}"
+                             })
+    errors.add(:bot_token, 'error setting up the webook') unless response.success?
+  end
+end