Improve cargo features

[EdJoPaTo]

#214 removes the implicit features of the dependencies.

Maybe the names can be improved / simplified (mentioning telegram in the telegram related crate is superfluous).

Maybe the default feature can be removed? Using the async client requires disabling the default features which is a bit annoying. Maybe always require adding the wanted feature?

[EdJoPaTo]

async-client and sync-client or client-async and client-sync?
Maybe go with the file names of the modules: client_ureq and client_reqwest and trait_async / trait_sync