Add "owner" to transaction policies

[AurelienFT]

Background:

Currently, each transaction can have many inputs with different owners. We currently have no way to have a "owner" and multiple implicit rules are used in the ecosystem.

Proposal:

To give information about a potential "owner" we propose to add a new transaction policy. This new policy would specify an index that will indicate which input of the transaction is considered as the "owner" of the transaction . This feature would enable developers to identify an initiator of the transaction.

Implementation Details:

Add a new optional field owner to the transaction structure.
Update the transaction validation logic to check that the index in the owner field exists in the transaction.

Example:

A transaction with the following policy:

inputs :

[
    Input::coin_predicate(
            UtxoId::new([0xaa; 32].into(), 0),
            [0xbb; 32].into(),
            Word::MAX,
            [0xcc; 32].into(),
            TxPointer::new(0x3802.into(), 0x28),
            Word::MAX,
            vec![0xdd; 50],
            vec![],
    ),
]

owner: 0

This transaction will provide the sender of the input 0 as the owner of the tx ([0xbb; 32]) and will be accessible in the code executed in this transaction