feat: Add maybe_async_trait

[igamigo]

Adds maybe_async_trait which adds a way to use async_trait based on whether the async feature is turned on. The attribute needs to be set for the trait impl and definition blocks. The macro takes functions annotated with #[maybe_async], makes them async, and then sets #[async_trait].
One small consequence of this is that when you have code like this:

use winter_maybe_async::{maybe_async_trait, maybe_async}

#[maybe_async_trait]
pub trait Example {
    #[maybe_async]
    pub fn foo() { }
}

If you turn on the async feature, then maybe_async becomes an unused import (because maybe_async_trait essentially removes it from foo) which might affect clippy lints. One way to deal with this would be to import macros like this: use winter_maybe_async::*.

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[igamigo]

Had to add various newlines and "summaries" (mostly placeholders, we likely need to replace with better text) to make the first paragraph shorter in doc comments because of failing clippy checks. For the same reason I also had to add a usage of div_ceil() instead of manually dividing.

[irakliyk]

Looks great! Thank you! I left a few comments inline - but these are for files not related to this PR. So, we can merge as is and then make updates separately.

[irakliyk]

All looks good! Thank you!