Support SOCKS5 proxy

[luisschwab]

Would be interesting to be able to pass a SOCKS5 proxy. Initially brought up on bdk#373.

[oleonardolima]

cACK, I guess it should be trivial by using the new bitreq/jsonrpc client, as it has a proxy feature.

[ValuedMammal]

jsonrpc has a proxy feature but only for the simple-http transport, whereas here we support bitreq. bitreq does have a proxy feature but evidently only does HTTP CONNECT, so no socks5.

I suppose one could bring their own http client configured with socks5 support, implement Transport for it, and go through bdk-bitcoind-client's Client::with_transport.

[luisschwab]

There is a PR to add SOCKS5 on bitreq: rust-bitcoin/corepc#533