Implement the `UtxoSource` interface for REST/RPC clients

Implement the `UtxoSource` interface for REST/RPC clients

In LDK, we expect users operating nodes on the public network to
implement the `UtxoSource` interface in order to validate the
gossip they receive from the network.

Sadly, because the DoS attack of flooding a node's gossip store
isn't a common issue, and because we do not provide an
implementation off-the-shelf to make doing so easily, many of our
downstream users do not have a `UtxoSource` implementation.

In order to change that, here we implement an async `UtxoSource`
in the `lightning-block-sync` crate, providing one for users who
sync the chain from Bitcoin Core's RPC or REST interfaces.