add support for esplora API

[remyers]

Should support esplora API (default to Blockstream.info).

In particular, the API calls to reproduce Samourai API:

POST /tx
GET /tx/:txid/status

And new functionality to support a basic Electrum client:

GET /tx/:txid/merkle-proof
GET /blocks[/:start_height]
GET /blocks/tip/height