Make BlockListener a public package for re-use

[peterbroadhurst]

This PR pulls the BlockListener out into a separate package for re-use, as part of indexing workflows.

This is a useful utility that tracks the head of an Ethereum chain, and keeps an in-memory canonical chain that extends back a pre-set number of "unstable" blocks (say 100).

If any changes are detected in this head, then the consumer is notified not just of the block, but of the consistent set of blocks ahead of that up to the new head. By processing this set of hashes as a single unit, rather than many individual events, logic that deals with confirmation management is able to be optimized.

The utility also allows multiple consumers within a single go process, to share a single stream of these changes.

Breakdown of proposed change

• Adding a pkg dir to firelfy-evmconnect
• Making the new ethblocklistener package available there
• Defining etherrors and ethrpc packages alongside for the core JSON/RPC data types and error mapping code
• Adding JSONFormatOptions to ethrpc so those structures can be serialized in more app friendly ways
  • Builds on abi.Serializer in firefly-signer
  • Provides a JSON Query style interface to build a single formatter
  • Same formatter can be used for formatting the JSON objects, as well as the ABI parsed data
• Minor refactors to interfaces between components
• Test enhancements and simplification