added implementation for json type

[edavidaja]

No description provided.

[machow]

Thanks for this PR! I'm going to tweak quickly, so it doesn't try to run the azure tests on external PRs.

[machow]

Note that a small difference between this implementation and R is that R uses jsonlite with the auto_unbox=TRUE option, so pins-python can roundtrip {"a": [1]}, but R pins will re-write it as {"a": 1}. (but this seems okay is more a challenge with everything being a vector in R).