Suggestion: use proto descriptor files as an alternative to plain protos

[KenxinKun]

A suggestion for v3: allow an alternative input in the form of proto descriptor files (i.e. that the user has already pre compiled with protoc).

Why? It's easy to get proto spec versions out of sync between what's supplied to gripmock and what's use in the codebase.

We use BSR imports into our Golang codebase and it's straightforward to pass in a proto descriptor from the imported package. On the other hand to pass the proto files to gripmock I need to reverse engineer what buf module version my Go package is importing and then use buf export. The chances of mis matching api versions is high.