[mlir] Add fallback support for parsing/printing unknown external resources

This is necessary/useful for building generic tooling that can roundtrip external
resources without needing to explicitly handle them. For example, this allows
for viewing the resources encoded within a bytecode file without having to
explicitly know how to process them (e.g. making it easier to interact with a
reproducer encoded in bytecode).

Differential Revision: https://reviews.llvm.org/D133460