synth-bench: inject the transactions bypassing the json-rpc nodes

[ssavenko-near]

Description

Current status

Synthetic benchmark spins up the json-rpc nodes to feed in the transactions into the system under test. That introduces an extra layer(s) resulting in the noise and may mask the bottlenecks in the Runtime system.

Goal

Tooling to inject the (valid, signed) transactions to the transactions pool bypassing the RPC layer.
Configurable TPS to check where we start dropping transactions.

Possible solution

Transaction producer running in the neard binary itself activated from the main config file ~/.near/config.json

Things to watch out

• minimize the impact of the transaction injection job on the Runtime
• cover much of the "normal" execution path apart from the RPC
• generating the valid transactions is not trivial cause the neard does not have the clients private keys