feat(sidecar): Create a delegations file format and load it on startup

[merklefruit]

We may need to load existing delegations on startup, and there currently isn't a format to do so.
Proposed solution:

A JSON file with the following format: Vec<SignedDelegation> to use our existing types.

[
  {
    "message": {
      "action": 0,
      "validator_pubkey": "0x...",
      "delegatee_pubkey": "0x..."
    },
    "signature": "0x..."
  },
  ...
]

At startup, the sidecar will attempt to load these delegations, and then it will cross-check the delegated pubkeys with the ones available in the provided signer (local, remote or via keystore files).

then the sidecar has all the info necessary to sign constraint messages when needed.