Enable type names for pbjson_types

[amfaber]

Hiya!

Thanks for the great work in making it easier to switch between protobuf and json / other serde encodings. I recently ran into an issue where I had to use the prost_types::Any type though. Prost's any provides some nice methods for going to and from prost::Message's, but they require the trait bound prost::Name. Name is not implemented on prost-generated types by default, but this can trivially be opted into by called .enable_type_names() on the config. An even more ergonomic change would be to also implement from_msg and to_msg on pbjson_types::Any, but simply deriving prost::Name on all the standard types atleast lets the end user use prost's Any by shuffing around the structs a bit.

Before merging, I believe a few things should be settled:

1. Is the feature even needed / wanted?
2. Should it perhaps be behind a feature flag? Since the additional generated code is such low volume, I estimate it to be fine to just have it on by default
3. Whether to also implement from_msg and to_msg on pbjson_types::Any