Customization of codecs

[amsminn]

Currently we have hard coded codecs for binary and JSON using standard marshaling APIs and no ability to customize.

It is important to be able to customize in some cases, such as passing registries for JSON any or extensions.

We should also consider similar for binary, and also see if it's possible to keep the google.protobuf dependency completely at the codec layer, allowing easily switching to other protobuf implementations. It may not be possible but let's explore the impact.

[anuraaga]

Thanks @amsminn - I have repurposed this issue for a higher level tracking of codec customization.

[amsminn]

That sounds great — I’d like to join the discussion and contribute.