Implement predefined field constraints

[jchadwick-buf]

Like protovalidate-go and protovalidate-java, we need to adjust the code to handle dynamic descriptor sets more robustly, since we need to jump between resolving the protovalidate standard rules and the predefined rule extensions. This necessitates adding a couple of additions to the API surface, namely ValidatorFactory::SetMessageFactory and ValidatorFactory::SetAllowUnknownFields, which controls instantiation of unknown dynamic types and whether or not to ignore unresolved rules, respectively. Like other protovalidate runtimes, we will default to failing compilation when unknown predefined rules are encountered. This should not break existing users but will prevent silent incorrect behavior.

TODO:

• Skip reparse when there are no empty fields—this way we can avoid pessimizing the common case
• Add an option to fail when unknown rule fields are unable to be resolved.
• Update for protobuf changes in Implement predefined field constraints protovalidate#246.

This will depend on bufbuild/protovalidate#246.

[jchadwick-buf]

Like with the other runtimes, we're currently blocked on updating the protos. That said, thanks to the wire compatibility of the updates, this PR should pass all of the relevant conformance tests in the new protovalidate version just fine. With that in mind, I would like to get the reviews started as soon as possible so we can keep things unblocked.

[jchadwick-buf]

Updated to Protovalidate v0.8.1, should be good to go.