Update field_mask.const expression to avoid descriptor confusion (#450)

#429 added rules for field
masks. In the rule `field_mask.const`, we're comparing two
`google.protobuf.FieldMask` messages against each other with the CEL
expression `this != getField(rules, 'const')`. This comparison is
susceptible to type confusion in some Protobuf / CEL implementations.

For example, protovalidate-go references the descriptor from
`fieldmaskpb` when providing the value to CEL, but the user may provide
a different instance of the field mask descriptor when validating a
message. This results in a false negative when the two messages are
compared by cel-go (see its behavior
[here](https://github.com/google/cel-go/blob/v0.26.1/common/types/pb/equal.go#L30)).

To mitigate the problem, this PR changes the CEL expression to compare
field mask paths instead of field mask messages.

Author: timostamm

proto/protovalidate/buf/validate/validate.proto

@@ -4706,7 +4706,7 @@ message FieldMaskRules {
   // ```
   optional google.protobuf.FieldMask const = 1 [(predefined).cel = {
     id: "field_mask.const"
-    expression: "this != getField(rules, 'const') ? 'value must equal paths %s'.format([getField(rules, 'const').paths]) : ''"
+    expression: "this.paths != getField(rules, 'const').paths ? 'value must equal paths %s'.format([getField(rules, 'const').paths]) : ''"
   }];
 
   // `in` requires the field value to only contain paths matching specified