Ability to set base URL in Telegram credentials

[ddzobov]

Summary

Ability to set base URL in Telegram credentials when we want to use Telegram Bot API behind reverse proxy.

Example cases:

• when n8n installation placed in networks without internet access, so we should use proxies queries
• when we want to use telegram bot routing application that runs over webhooks
• when we want to track all queries to telegram and review/decorate them in some cases
• when we don't want to share original telegram bot tokens to applications and replace them with custom tokens

Example endpoints:

• https://api.telegram.org/bot - default, public
• https://localhost:9000/bot - custom, proxied

Related tickets and issues

#2517

Review / Merge checklist

• [x] PR title and summary are descriptive. Remember, the title automatically goes into the changelog. Use (no-changelog) otherwise. (conventions)
• [ ] Docs updated or follow-up ticket created.
• [ ] Tests included.

  A bug is not considered fixed, unless a test is added to prevent it from happening again. A feature is not complete without tests.