[Feature Request] Allow complex types for value of `rule` CEL variable.

[jmmills]

Feature description:

Currently the value of the rule variable seems to only work with primitive types rather than custom messages

Works:

extend buf.validate.StringRules {
  optional string allow = 536870909 [(buf.validate.predefined).cel = {
    id: "string.identifier"
    message: "field must be a valid identifier"
    expression: "this == rule"
  }];

Does not work:

extend buf.validate.StringRules {
  optional ComplexRule allow = 536870909 [(buf.validate.predefined).cel = {
    id: "string.identifier"
    message: "field must be a valid identifier"
    expression: "rule.enabled && this == rule.value"
  }];

message ComplexRule {
    optional bool enabled = 1;
    optional string value = 2;
}

Problem it solves or use case:
This would allow more complex rules to be written in a cleaner fashion.

Proposed implementation or solution:
When the value of the predefined rule type is a proto message supply it as the value for rule.

Contribution:
I'm willing to attempt a PR if the maintainers don't have any deeper reason why this should not be allowed.

[timostamm]

I would expect your example to work. Can you explain the problem in more detail? Which protovalidate implementation are you using? What's the error message?