Add support for rapidjson as the JSON library

[ben-clayton]

nlohmann is still the default, but if CPPDAP_JSON_DIR points to rapidjson, then this will be used instead

Most of this was upstreamed from:
https://fuchsia-review.googlesource.com/c/third_party/github.com/google/cppdap/+/456566

Fixes: #49

[amaiorano]

Long-term, it feels like rather than writing wrappers around different JSON libs, it might make more sense to allow clients of the library to write their own wrappers, say by deriving from the common base, and providing it to cppdap via some factory function or something.

Also, it looks like there are no tests to validate that both libs work as expected, unless I missed it?

[ben-clayton]

Long-term, it feels like rather than writing wrappers around different JSON libs, it might make more sense to allow clients of the library to write their own wrappers, say by deriving from the common base, and providing it to cppdap via some factory function or something.

I agree, but I'd rather not add support that isn't currently needed. If we get a request to do this then it should be pretty straightforward to do.

Also, it looks like there are no tests to validate that both libs work as expected, unless I missed it?

Yes, I need to add presubmit checks to do this. Its on my TODO list (it's been a really heavy couple of weeks). I'd like to get this landed though as it unblocks fuchsia.

[ben-clayton]

Thanks. Created #53 so this isn't forgotten.