Add basic event stream RPC interface docs and JSON/RPC client lib

[peterbroadhurst]

In PR chain with #515

• Adds documentation
• Consolidates rpcbackend JSON/RPC client from firefly-common into rpcclient to allow us to enhance it
  • Also has the benefit of eliminating lots of type mapping complexity
• Enhances Subscribe() functionality of rpcclient.WSClient to support:
  • ptx_subscribe("receipts", "listenerName")
  • ptx_ack
  • ptx_nack

[peterbroadhurst]

Putting back into draft to add to E2E test for full validation

[peterbroadhurst]

Added E2E test, and worked through:

• Mismatch in nesting of result in the payloads by convention with Ethereum notifications
• Not attempting to post-filter on sequence, as the pre-insertion trigger does not have access to the sequence

[peterbroadhurst]

Attempted to drive one step closer to consistency with eth_subscribe:

paladin/core/go/internal/txmgr/rpc_eventstreams.go

Lines 155 to 167 in 39a7450

	// Note we attempt strong consistency with etH_subscribe semantics here, as described in https://geth.ethereum.org/docs/interacting-with-geth/rpc/pubsub
	// However, we have layered acks on top - so we're not 100%.
	// We also end up with quite a bit of nesting doing this:
	// { "jsonrpc": "2.0", "method": "ptx_subscription",
	// "params": {
	// "subscription": "0xcd0c3e8af590364c09d0fa6a1210faf5",
	// "result": {
	// "batchId": 12345,
	// "receipts": [ ... interesting stuff ]
	// }
	// }
	// }
	sub.ctrl.Send("ptx_subscription", &pldapi.JSONRPCSubscriptionNotification[pldapi.TransactionReceiptBatch]{

[hosie]

LGTM. I left a few comments on #515 but nothing worth holding this out.