signature: reconsider `allow(async_fn_in_trait)`?

[tarcieri]

I'm curious why we're suppressing this lint. The reason given in #1419 which did the original AFIT migration is:

#[allow(async_fn_in_trait)] is required until a solution to RPITIT is merged in rust.
This put responsability on the implementor of async_signature::AsyncSigner to make sure their future is Sendable.
see rust-lang/rust#115822 (comment) for more context.

RPITIT shipped in 1.75.0, so should we reconsider this?

cc @baloo

[tarcieri]

I guess this is another area where I'd just like to ship signature v3 and lacking other feedback to the contrary, I'd prefer not to make big changes at the last minute. So I'd prefer to leave things as-is.

If need be, we can re-evaluate this in signature v4.