Make sponsorship note identification forward-compatible

I wonder if we can make this more flexible later, for the case where we have multiple variants of sponsorship notes (e.g. the standard one, but also user-defined).

The defining characteristic of a sponsorship note is not its script, but its functionality: it carries assets that can be moved to the vault, and does basically very little else. But "very little else" is arguably hard to capture in code...

_Originally posted by @mmagician in https://github.com/0xMiden/protocol/pull/3320#discussion_r3597234226_