Add support for custom API server in Telegram integration
What
Following the #441097 (comment 1885036030), we should add a support for custom API server in our Telegram integration, https://core.telegram.org/bots/api#using-a-local-bot-api-server.
Proposal
Add a new non-required hostname
field, and if it's not provided, use https://api.telegram.org
as a default value.