Add Target onion_decode

[erickcestari]

Custom Mutator Development

• Integrate secp256k1 as a library within the custommutator directory for cryptographic operations
• Implement a custom mutator that performs the following workflow:
  1. Decrypt the onion packet
  2. Mutate the raw payload data
  3. Re-encrypt the modified packet

Target Implementation

• Create fuzzing targets for Core Lightning, LDK and LND implementations

Future Work / Follow-ups

• Improve the custom mutator so that it can create valid TLV onion payloads more often.
• Update the custom mutator to generate blinded onion TLV payloads.

Closes #306 Replace ##323

[erickcestari]

I think we could remove all the unnecessary stuff from secp256k1 (e.g. .github, CHANGELOG, CONTRIBUTING, etc)

I have removed those now!

[brunoerg]

Needs rebase. What's the status of it?

[erickcestari]

Needs rebase. What's the status of it?

Edited: I actually think it's ready to go. Since this is already a significant PR, it would be better to make the changes in follow-up PRs.