Add support for shared field constraints

[jchadwick-buf]

There doesn't seem to be any practical way to either globally register extensions or have CppProtobuf recursively handle option extensions through a chosen MessageFactory/DescriptorPool so we wind up needing to push the logic to reparse dynamic extensions all the way down to the bottom when the rules structure is evaluated. This adds a small addition to the API surface, ValidatorFactory::SetMessageFactory, so that the end users (and conformance runner) can set a descriptor pool to use for reparsing.

TODO:

• Update for protobuf changes in Implement predefined field constraints protovalidate#246. Has not been necessary for testing because the protos/extensions are backwards compatible.
• Skip reparse when there are no empty fields—this way we can avoid pessimizing the common case (though the reparse is quite negligible in most cases.)

This will depend on bufbuild/protovalidate#246.