Introduce loadtest Command to Soroban RPC

[stellarsaur]

What

This PR adds a new loadtest command to generate configurable load to a running Soroban RPC server.

Examples:

Simple ping loadtest at 10000 requests/s for 1 minute
soroban-rpc loadtest --soroban-rpc-url=<url> -g getHealth -n 10000 -d 1m

Simulate transaction loadtest at 1000 requests/s for 3 minutes
soroban-rpc loadtest --soroban-rpc-url=<url> -g simulateTransaction -n 1000 -d 3m --network-passphrase="Test SDF Future Network ; October 2022" --hello-world-contract-path="./test_hello_world.wasm"

Tested by spinning up a Soroban RPC docker image on a devbox and generating load to it from my local machine.

Why

It's useful for both us, and infrastructure providers, to be able to benchmark soroban-rpc performance with different types of request loads. In the future, we can consider productionalizing this to run on soroban releases to make sure we don't introduce notable performance regressions.

Known limitations

• Limited types of requests able to be generated currently (getHealth, getEvents, simulateTransaction)
• Config only from CLI and not config file
• Manually have to inspect resource consumption on server machine (soroban-rpc metrics have limited support)

[stellarsaur]

Closing this since all new RPC contributions belong in the soroban-rpc repo. If we want to move forward with incorporating this command I'll reopen this PR in the soroban-rpc repo.