Add field and rule value to violations

[jchadwick-buf]

Adds the ability to access the captured rule and field value from a Violation.

C++ protobuf doesn't really have a native "value" representation unlike other languages, so I've created a sort-of stand-in. The ProtoField class acts as a reference to a field on a message, and provides an interface to get a singular value as a variant. This should be good enough for most users, but users can also grab the information needed and use protobuf reflection directly instead if they have more complicated needs.

This is a breaking change. The API changes in the following ways:

• Validator::Validate() now returns the wrapper type ValidationResult instead of Violations. This can be converted into the protobuf Violations form using the proto() method.
• ValidationResult.violations() returns a ConstraintViolation. The underlying Violation can be accessed using the proto() method.