Add support for supplying protojson Marshal options to json codec.

[jmmills]

Is your feature request related to a problem? Please describe.
For certain cases of javascript ecosystem interoperability it may be required to control certain behaviors for how json is rendered. protojson offers this via the MarshalOptions

Describe the solution you'd like
Offer the ability to either inject codec specific options via middleware and/or set default configuration on service startup.

Describe alternatives you've considered
Implementing a custom codec that does the same thing as the canonical json codec with the added ability to inject marshal options.

Additional context
It seems like the most useful options from the perspective of how data is rendered are EmitUnpopulated, EmitDefaultValues

[jhump]

We've gotten a lot of feedback that folks would like to see that functionality in this core package and are taking it under consideration. In the meantime, there is a high-quality implementation of what you're looking for in this 3rd party package: https://github.com/akshayjshah/connectproto

[TomoyaOnishi]

I’ve also been eagerly waiting for this option.
While implementing numerous API endpoints, I’ve faced many challenges due to the inability to represent key: null. Adding this feature would allow for more flexible support of data structures and significantly improve interoperability with other ecosystems.
I truly hope this will be considered!

[emcfarlane]

Solved in the v2 proposal (#951). Codecs moved into their own package, connectproto, and the JSON codec exposes the protojson options directly as public fields:

codec := connectproto.NewJSONCodec()
codec.MarshalOptions.EmitUnpopulated = true

Register it with connecthttp.WithCodec on the client transport or on connecthttp.Mount for a server, and connecthttp.WithConditionalOptions can scope it to specific procedures.