Add access to custom options (of host messages).

[yangzh]

Feature description:
Thanks for this great idea and great library!
I would like to see the custom option (of host messages) to accessible by protovalidate / CEL.

For example, if I have a .proto file:

extend google.protobuf.EnumValueOptions {
string alt_name = 10000;
}

enum PartOfSpeech {
UNKNOWN = 0;
NOUN = 1 [(alt_name) = "noun"];
VERB = 2 [(alt_name) = "verb"];
etc.
}

message Token {
string pos = 1 [(buf.protovalidate).string = {
in_enum_values: "alt_name" }];
}

so that the validation rule basically limit the value of "pos" to be one of "noun", "verb", etc. (as long as we supply alt_name for each enum value.

This particular case might be trickier (than what's required by this feature), it will have to somehow aggregate the alt_name from all enum values, and use that set as the target for "in" operation: you get the idea.

But in general, I want to access to the custom option as supplied by original protobuf authors, in protovalidate, or more generally in CEL.

Proposed implementation or solution:
I guess beside binding "this" into the evaluation context, you need to somehow make the custom options (of the host message) accessible from inside the evaluation context as well.

Contribution:
I don't have time to implement, but I'm happy to elaborate and help testing the feature.