add JSON module and enable JSON serialization conformance tests

[andrewparmet]

No description provided.

[friscoMad]

Hi @andrewparmet, first of all thank you for all the hard work in this lib and sorry to ping you here but my company is considering using protoKT for our stack and I would like to know if you feel that JSON and protovalidate integration are planned to be merged "soon", both features + #278 would be very valuable for us.
If we end up deciding going with ProtoKT I have no problem spending some time and helping with the implementation.

[andrewparmet]

@friscoMad a few notes:

• The JSON module introduced in this PR is a formality wrapping behavior you can implement yourself if you need it more urgently. Simply make a copy of https://github.com/open-toast/protokt/pull/219/files#diff-b99e1237f1eae191c6e42040e31a8d5ea2bf24d2ae3e2c9bbf2d53a28d59e94d for yourself.
• Protovalidate integration is more complex but I anticipate it being merged sooner - but note that you can copy that for yourself as well.
• I'll address Add an option to use converters for all well known types #278 there.

[friscoMad]

Thanks for the responses @andrewparmet

• Wow I did not check the JSON implementation other than seeing a lot of commits and a dozen file changes, but it is great that it is a one-liner.
• I was assuming that for protovalidate we need first to get Introduce MessageReflector to enable alternative protobuf runtimes bufbuild/protovalidate-java#132 this merged but I understand now that that is only required to get some perf updates.