Abstract away the need for setting witness indexes

[MujkicA]

Currently, our Input type requires you to set a witness_index. This is propagates to the methods get_asset_inputs_for_amount, add_fee_resources which is especially painful because they are defined on the Account trait. Predicates as an implementor of Account have no use for the witness_index.

witness_index is the index in the witnesses field of a transaction where the signature of the wallet that owns the input is expected to be found. It's quite tricky to avoid exposing this on a higher level as we can't ignore the order of signatures because every signer also signs over the witnesses field and thereby includes every previous signature.

@segfault-magnet mentioned the possibility of storing calls to sign_transaction as intentions which will be applied in a post-processing step, similarly to how we handle dynamic data encoding for predicates.

Another option might be to always default to idx 0 (or 1 in the case of Contract txs) and have a special interface for creating txs with multiple signatures since they are more of a niche use-case.