refactor: use RPITIT over async_trait

[Nerixyz]

Towards #393. This removes the dependency on async_trait and uses RPITIT in Client and TwitchToken. Using async fn in the trait definition is not enough, because most executors want futures that are Send.

For the ReqwestClient, I kept the constraint that the returned future doesn't capture the lifetime of the client. TwitchToken and its implementations captured 'self and the client's lifetime already.

[Nerixyz]

Not happy about having to bump the MSRV again because of + use<...>, but we need that to avoid capturing too many lifetimes if I understand correctly.