Improve JSON support in Esqueleto.Postgresql

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.