feature: expose protobuf export tools

[kyessenov]

Downstream projects would like to be able to export CEL values to a protobuf serialized form.
This should be a generic function provided by the runtime.

[kyessenov]

@JimLarson @TristonianJones

In Envoy WASM ABI, we have a function to serialize a CEL value across the boundary. For native types (ints) we use a native representation (bits). But for maps and lists, I wrote a custom serializer to a protobuf value, and then proto marshal that. That generally works, but it's a bit confusing in some places:

• non-string keys in maps
• message values - should they always be converted to protobuf struct or use native message serialization