Add signature validation tool + fix checkpointing padding.

[picnoir]

Description

We add a signature validation tool allowing the OBFT team to create a automated integration test checking the signatures issued by Morpho.

We also pad the ECDSA Signature bigInt2Bytes output.

Note: this has been tested using the Morpho integration test suite:

https://hydra.project42.iohkdev.io/log/lak4pp2cy1ych2kzzm5d7lr4dx7jnmv4-checkpointing-node-mantis-integration-tests.drv

unpacking sources
unpacking source archive /nix/store/0zyf6r2rlypf8la9prn6ks1105cz9hww-source
source root is source
patching sources
configuring
no configure script, doing nothing
building
no Makefile, doing nothing
installing
Mantis Integration Tests
  mantis-integration
    Test the keygen against the actual Mantis parser: OK (136.19s)
      +++ OK, passed 100 tests.

All 1 tests passed (136.19s)
@nix { "action": "setPhase", "phase": "fixupPhase" }
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/7aliz8s7ipfzmbhsssrlhbhvrcpjg45v-checkpointing-node-mantis-integration-tests
patching script interpreter paths in /nix/store/7aliz8s7ipfzmbhsssrlhbhvrcpjg45v-checkpointing-node-mantis-integration-tests
checking for references to /build/ in /nix/store/7aliz8s7ipfzmbhsssrlhbhvrcpjg45v-checkpointing-node-mantis-integration-tests...

[ntallar]

LGTM!

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I can't avoid marking this comment, but it's not a blocker for merging

[ntallar]

We should probably move this decoder to ByteStringUtils and not rely here on the jsonrpc methods

[aakoshh]

By the looks of it ByteUtils.bigIntToBytes should have been doing the same thing, except it was lacking padding on the left. Unfortunately it doesn't have any docstrings or tests, so it's difficult to tell what it's supposed to be doing. I know that some private keys generated by Secp256k1 are 33 bytes and some are 32 when converted to a BigInt, which is why I had to change serialization here. If this change made a difference in your case, does it mean you had a BigInt which had a different length? Do you have examples? Should ByteUtils.bigIntToBytes also do padding? Could you add unit tests?