Fix update model schema

[pershinmr]

Per https://core.telegram.org/bots/api#update only one of the field is present - so currently update with edited_message breaks the Pydantic model schema validation:

ERROR:root:1 validation error for Update
message
  Field required [type=missing, input_value={'update_id': 222768138, ...type': 'bot_command'}]}}, input_type=dict]

as message field is not optional.

This PR addresses this.