impl Message for serde_json::Value

[danburkert]

For any application utilizing both protobuf and serde, it'd be better to use serde_json::Value in place of prost_types::Value. I believe (but haven't tested) that it's possible to get prost-build to output serde_json::Value instead of prost_types::Value using prost_build::Config::extern_path(".google.protobuf.Value", "::serde_json::Value"), however ::serde_json::Value doesn't impl Message, so this won't currently work.

The solution is to add an optional serde_json dependency to prost and conditionally add the impl.

[danburkert]

and/or perhaps add reflexive From impls to convert prost_types::Value <-> serde_json::Value

[archseer]

#181 was an attempt at that.