util: PollSemaphore should expose available_permits

[hawkw]

Is your feature request related to a problem? Please describe.
The tokio::sync::Semaphore type provides a Semaphore::available_permits method which returns the current number of permits available on the semaphore. tokio_util::sync::PollSemaphore does not expose such a method. It is possible to use PollSemaphore::into_inner or PollSemaphore::clone_inner to unwrap the inner semaphore, but this may require cloning/dropping the semaphore's Arc which shouldn't be necessary to access the permits.

Describe the solution you'd like
We should add a PollSemaphore::available_permits that calls Semaphore::available_permits on the inner Semaphore. We could probably also add a PollSemaphore::add_permits method while we're here. This should be pretty trivial to add.

Describe alternatives you've considered
Alternatively, we could just add a PollSemaphore::inner_ref or similar method for borrowing the inner Semaphore. PollSemaphore could even implement AsRef<Semaphore>.

This might be a little more complex to use (semaphore.as_ref().available_permits() vs semaphore.available_permits()) and is less visible in the API docs, but it would let us easily expose all the methods on the inner type transparently without having to wrap them. This could be both a pro or a con, if there are methods of the inner Semaphore type that PollSemaphore should not expose...

[Darksonn]

I thought we already had an impl AsRef<Semaphore> for PollSemaphore, but I see that it is missing. It would be nice to add that too.