Fix bug where cel expression cannot compile for fields of type google…

….protobuf.Any (#65)

This fixes the problem where cel expressions defined on fields of type
`google.protobuf.Any` cannot compile.

For example, when validating a `FieldOfTypeAny` with its `any` field set
to a `Simple`, it would fail with
```
runtime error: error evaluating any_type: unknown type: 'tests.example.v1.Simple'
```
```
message Simple {
  string s = 1;
}

message FieldOfTypeAny {
  google.protobuf.Any any = 1 [(buf.validate.field).cel = {
    id: "any_type",
    message: "this should never fail",
    expression: "this == this"
  }];
}
```

internal/evaluator/builder.go

@@ -274,6 +274,7 @@ func (bldr *Builder) processFieldExpressions(
 	var opts []cel.EnvOption
 	if fieldDesc.Kind() == protoreflect.MessageKind {
 		opts = []cel.EnvOption{
+			cel.Types(dynamicpb.NewMessage(fieldDesc.ContainingMessage())),
 			cel.Types(dynamicpb.NewMessage(fieldDesc.Message())),
 			cel.Variable("this", cel.ObjectType(string(fieldDesc.Message().FullName()))),
 		}