Add subcrate which impls a simple SPV client from Bitcoin Core RPC

Add subcrate which impls a simple SPV client from Bitcoin Core RPC

This adds a new subcrate `lightning-block-sync` which is designed
to make it easier to get up-and-running by removing the effort of
building an SPV client and fetching the chain.

Instead of building a P2P client (and all the address management
that entails), this focuses on building a trivial SPV client which
can fetch from several instances of an abstract BlockSource. Then,
we provide two example BlockSource implementations that can fetch
from Bitcoin Core's RPC interface and Bitcoin Core's REST interface.

The code here is taken with heavy modifications from
rust-lightning-bitcoinrpc.