Improve JSON support in Esqueleto.Postgresql

[belevy]

As a user I would like to be able to extract JSON values from a query. This would be useful for faking multiset support similar to jOOQ.

Proposed Additions

• Add a new JsonValue newtype wrapper with a SqlSelect based on JSON parsing
• Add support for to_json on SqlExpr (Value a) and SqlExpr (Entity a) and tuples (up to 8?)
  • Value a is trivially implemented using the built in to_json
  • Entity a can use the entityDef to json_build_object assuming that the FieldDef.fieldHaskell matches the default persistent json
  • Tuples can be represented using json_build_array
• Add support for json_agg :: SqlExpr (JsonValue a) -> SqlExpr (JsonValue [a]) this is limited to JsonValue to enforce the use of our to_json implementation instead of relying on the default postgres behavior.

[rjmholt]

Given #284, it might be worth closing this issue as resolved

[belevy]

Not sure what #284 has to do with this issue? #283 is the PR associated with this issud