Support pack / unpack of google.protobuf.Any values

[TristonianJones]

Issue Filing Checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request

Change

The CEL language definition indicates that google.protobuf.Any values are automatically
packed / unpacked to their underlying proto message during expression evaluation. At present,
the interpreter does not handle this.

Example

If msg is declared as a message containing a field any of type google.protobuf.Any, the
value of any could be any protobuf message type supported by the types.Provider and
it should be automatically converted to this underlying type

// treat msg.any as a Timestamp when it is a google.protobuf.Timestamp
msg.any < timestamp("2017-01-01T00:00:00Z")