Add support for more custom params (existing key) in swarm-test `new_ephemeral` functions

Description

First of all excuse me if I am missing something, this is a suggestion and a question as well.

libp2p-swarm-test are a great way to test libp2p protocols. However there is only a very specific implementations of
``fn new_ephemeral` variants, that create keys inside the function.

There is factory method which helps but still sometimes I want to create a Swarm with specified keys.

Motivation

I am using libp2p-swarm-test in my tests, specifically:
fn new_ephemeral(behaviour_fn: impl FnOnce(libp2p_identity::Keypair) -> Self::NB) -> Self functions (I use tokio variant).
In lot of tests I actually need to create Swarm with existing key, and maybe specify some additional parameters to use in in creating a Swarm. I understand that the doc clearly says that the user of this functions should not rely upon the underlying parameters, but still the existing key use-case remains for me.
I solved this problem by creating my own Swarm factories with MemoryTransport but I was wondering if it would make sense to expand the existing ones in libp2p-swarm-test as well, as I really like the approach around libp2p-swarm-test.

Current Implementation

Current implementation supports most use-cases but does not support specifying already created key to a new_ephemeral function.

Are you planning to do it yourself in a pull request?

Yes

Start by locating the libp2p-swarm-test new_ephemeral variants, especially the tokio form that accepts a behaviour function. Compare those constructors with the existing factory and MemoryTransport approach, then define support for an existing libp2p_identity::Keypair and any additional swarm parameters; done means the requested configuration can be used without a custom factory.