Expose a trait impl'd for all `PeerManager` for use as a bound

Expose a trait impl'd for all `PeerManager` for use as a bound

A while back, in tests, we added a `AChannelManager` trait, which
is implemented for all `ChannelManager`s, and can be used as a
bound when we need a `ChannelManager`, rather than having to
duplicate all the bounds of `ChannelManager` everywhere.

Here we do the same thing for `PeerManager`, but make it public and
use it to clean up `lightning-net-tokio` and
`lightning-background-processor`.

We should likely do the same for `AChannelManager`, but that's left
as a followup.