Omnichannel Telegram blacklist support

[tiaga]

Is your feature request related to a problem? Please describe.
It's not possible to block a Telegram user in Rocket.Chat Omnichannel (Telegram integration), thus a user can easily flood you with messages.

Describe the solution you'd like
An ability to block Telegram users on Rocket.Chat Omnichannel side (for instance in Telegram integration settings) to ignore messages from them.

Describe alternatives you've considered
None

Additional context
None