Add wsbackend client

[nguyer]

This PR adds a JSONRPC WebSocket client.

• Provides sync CallRPC functionality over an async WebSocket transport
  • Manages JSON/RPC request IDs for you
  • Provides automatic reconnect, and all the other quality of life benefits of the firefly-common WebSocket impl
• Supports eth_subscribe/eth_unsubscribe with a higher level Subscription semantic
  • Automatically resubscribes if your WebSocket reconnects
  • Has a client-side ID to track your subscription, as well as the server-side ID (which changes on reconnect)
  • At-most-once delivery semantics is all you get with raw Ethereum WebSockets (it's all the eth_ protocol supports) - you miss it, you lose it
    • If you need more than that, use the full FireFly core which provides reliable check-pointed subscriptions with a higher level blockchain connector

[codecov-commenter]

Codecov Report

Merging #50 (7e346ca) into main (be29668) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main       #50    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           34        35     +1     
  Lines         2707      3015   +308     
==========================================
+ Hits          2707      3015   +308     

Files	Coverage Δ
pkg/ethtypes/hexinteger.go	100.00% <100.00%> (ø)
pkg/rpcbackend/backend.go	100.00% <100.00%> (ø)
pkg/rpcbackend/wsbackend.go	100.00% <100.00%> (ø)

[peterbroadhurst]

Adding my approval as code owner in this repo - but also asked @matthew1001 to take a look as @nguyer and I have now both made significant contributions