Improve MPTWG's witness output

[ChihChengLiang]

Describe the feature you would like

Asserting the current MPT witness generator's correctness is hard. The only path I see is through

1. MPTWG generates witnesses from Infura or Geth.
2. MPTWG dumps json files, which serves as test cases for the MPT circuit.
3. The MPT circuit reads json files and runs mock prover

We have completed step 1 of #1566, which ported MPTWG to this codebase. Now we are at step 2.

JSONs are only generated in the test run and passed directly to the circuit. We make sure the MPT circuit consumes the correct version of witness automatically.

But there's a problem, if the json is generated from a mainnet block, it might be better to store the json file, so that we don't depend on Infura on every test run.

But whether we decide to keep json files or not. It would be great to design for format for it for readability.

JSON format

• The first thing to do is to reduce the space occupied by the bytes. We encode them in hex strings.
• prettify the struct for readability.

Additional context

No response