Finalize `constraint` schema

[mempirate]

Context

The current constraint schema and API spec deviates a bit from what we originally had in mind in the following ways

• The constraint_proofs field should be a part of message when submitting a block and forwarding a signed header. The signature should be over the whole message field.
• Naming should be consistent over all components, i.e. constraint_proofs

Typing

We should also make the schema more future proof by supporting typed constraints and proofs. For inclusion proofs, this could look like:

{
    "type": 0, // type for inclusion proofs
    "data": {
        "generalized_indexes": [
            4,
            5
        ],
        "merkle_hashes": [
            "0x5097...",
            "0x932587..."
        ]
    }   
}