Add postgres JSON cast type

[wexder]

Is your feature request related to a problem? Please describe.
We are using a lot of json fields in our postgres database. And it's hard to do the UPSERT with json column type.
Currect solution I found is double casting the json string.

  insertStm := table.INSERT(table.AllColumns).MODEL(table).
  ON_CONFLICT(table.SomeColumn).DO_UPDATE(
  postgres.SET(
    table.JsonColumn.SET(postgres.StringExp(postgres.CAST(postgres.String(*data)).AS("json"))),
    ),

Describe the solution you'd like
Add postgres.Json() func that would return StringExpression.

[wexder]

I've create PR for this #161

[go-jet]

Hi @wexder. Yeah, it makes sense to have a json literal.

[go-jet]

Support added with v2.9.0 release.