Feature Request: Automatic update filtering

[WaffleLapkin]

Both GetUpdates and SetWebhook methods accept the allowed_updates parameter which allows filtering updates that are not needed. Teloxide should automatically check which handlers are set up, and set allowed_updates to an appropriate value.

I propose adding the following method to the UpdateListener trait:

fn hint_allowed_updates(&mut self, hint: impl IntoIterator<Item = AllowedUpdate>);

(cc @Hirrolot should I do this in #385, or should I make a new PR for that?)

Pros

Users won't need to provide allowed_updates by themself and so won't make a mistake providing allowed_updates with redundant update types or without wanted update types.

Cons

None known.

[Hirrolot]

LGTM.
I think we should do it in a separate PR since this is quite different functionality.