Shift the message signature profile to a more specific type.

[mikewest]

https://wicg.github.io/signature-based-sri/#profile requires that the type parameter of a conformant signature-input field have a value of sri. It probably makes sense to require something more specific, as I can easily imagine future changes to this scheme that would fit into the "sri" umbrella, but would have different specific characteristics (even something as simple as changing the signature algorithm).

ed25519-provenance, ed25519-integrity, ed25519-resource-integrity, etc. all might be better options.