Allow passing a NetworkBehaviour trait implementation to customize the networking

[tomaka]

While sc-network now has a easy-to-use gossiping API (#4284), there are situations, such as substrate-lfs, where you want to have more low-level control over the networking.

Libp2p provides a NetworkBehaviour trait that does that and that is designed to be combined with other NetworkBehaviour.

We should add a way for a user to "inject" a NetworkBehaviour into sc-network.
Substrate already implements its own NetworkBehaviours, and they would then be combined with the one provided by the user.

[tomaka]

The road to that is to first get libp2p/rust-libp2p#1405 in libp2p.
While it seems like purely a convenience, it is actually essential if we don't want to add hundreds of lines of unmaintainable trait bound requirements throughout Substrate.

[tomaka]

Since we're aiming towards stabilization, and that the libp2p NetworkBehaviour would be too much of a burden to expose on the API level, the plan now is to let users customize the networking with notifications protocols they register and request-response protocols they register.

The former is already possible. The latter is #5536. As such, closing this issue.