Esplora HTTP REST API Server Support

[cryptoquick]

Hi, I'm currently adding HTTP server support for Electrs: romanz/electrs#911
This is based on the Blockstream Esplora API: https://github.com/Blockstream/esplora/blob/master/API.md#esplora-http-api
I'm doing this to better support self-hosted Bitcoin node solutions like the Start9 Server for BitMask Wallet.

While communicating with the S9 team, they floated that this work might also solve a problem they'd been having with their Sparrow wallet users.

Apparently Sparrow cannot make LAN calls to Electrs because HTTP is the only protocol StartOS can use on LAN.

So, adding HTTP server support to Electrs could open the door to solving this problem for Sparrow users, so long as Sparrow has the capability to make calls to the Esplora HTTP REST API.

Also, if the client supports HTTP2, it might have the potential to be faster than separate TCP JSON-RPC calls.

[craigraw]

Apparently Sparrow cannot make LAN calls to Electrs because HTTP is the only protocol StartOS can use on LAN.

@gStart9 can you confirm this?

[gStart9]

It's true at the moment. We so far only have HTTP(S) reverse proxy capabilities into services' containers from the LAN. Regular TCP port forwarding is waiting for our LAN-port-forwarding-to-the-containers feature coming in our next major release (v040).

It's news to me that someone made a rest interface for its RPC, that's awesome!