Feature Request: make requester_forward macro public

[DCNick3]

Expose requester_forward macro, which can be used to implement adapters in 3rd party crates with less maintenance.

I, myself, am looking into this to implement adapter for tracing the requests and their responses as tracing spans (instead of just log messages, as does the adapter included upstream)

(discussed previously at #821)

Pros

• Allows the user to implement their own bot adapter without much boilerplate and without the requirement to maintain their own macros for this

Cons

• A bit larger maintenance burden, wouldn't be able to change it w/o a breaking release

Alternatives

Include tracing adapter upstream. No real alternatives for general 3rd-party adapter development