Make ClientBuilder directly accept boxed handler types

[DPlayer234]

Rather than having the ClientBuilder accept impl Into<Arc<H>> where H: HandlerTrait, directly accept the corresponding Arc<dyn HandlerTrait>.
This allows accepting non-concrete handler types (f.e. if constructed by a function that returns Arc<dyn HandlerTrait> already) and fixes type inference failures when passing an Arc<H> rather than H.

This affects the setters for EventHandler, RawEventHandler, Framework, and VoiceGatewayManager.

Framework is in a bit of a weird spot because it is only put in an Arc after Framework::init has been called, so a &mut can be taken to it for that call, so this PR has ClientBuilder accept Box<dyn Framework> for it.

Most of the PR diff is related to fixing examples.

[Joshix-1]

the readme mentions the event_handler method. You should update that and check if there are other markdown files mentioning the methods you changed

[DPlayer234]

The example in the README was horribly outdated, I've replaced it with a trimmed down version of the up-to-date ping bot example.
Didn't find any other places where these functions are used in non-Rust files.