Second stab at adding JSON support

[m-o-e]

Following the discussion in #22 (comment)
this adds to_json and from_json to all generated message-classes.

It works surprisingly well for me (even with nested objects etc.)
but the following caveats apply:

• It won't compile if you have messages that contain fields of type bytes.
  Should probably not be merged or be put behind an env-toggle until that can be addressed.
• I don't know if the output complies with the Proto3 canonical JSON encoding - but I think that's not necessarily a blocker (JSON support can be very useful even without interoperability with other impls).

[jgaskins]

• It won't compile if you have messages that contain fields of type bytes.
  Should probably not be merged or be put behind an env-toggle until that can be addressed.

Based on the link you gave to the JSON-encoding spec, this should actually be pretty trivial to solve if bytes is the only type it can't handle:

If we add a @[JSON::Field(converter: ::Protobuf::JSONBytesSerializer)] annotation to all bytes fields, we'll only need to implement JSONBytesSerializer module to automatically handle conversion to and from base64.

• I don't know if the output complies with the Proto3 canonical JSON encoding (it probably doesn't)

That link to the spec should make it easy to write up some tests for this. 💯

[m-o-e]

Before this change:

FooBar.from_json # => { oo_a: 1, oo_b: nil, my_oneof: nil }

After this change:

FooBar.from_json # => { oo_a: 1, oo_b: nil, my_oneof: "oo_a" }

Tested it manually a bunch and it appears to work reliably also with nested objects.