Adding JSON serialized Array columns

[jwoertink]

Fixes #845

This PR allows you to convert your json arrays in to an array of serialized objects.. One thing to note about this PR is that it's only supporting '[]'::jsonb and not []::jsonb[]. The distinction here is that the column is still stored as jsonb, but it allows that json string to be parsed as an array VS using postgres's ARRAY column to store an array of jsonb objects.

I think we could eventually do that one too by just saying that the column isn't serialized, but that can be for a future PR.

With this, you can now do something like:

struct Server
  include JSON::Serializable
  property host : String
end

class Monitor < BaseModel
  table do
    column input_server : Server
    column output_servers : Array(Server)
  end
end

in_server = Server.from_json(%({"host":"x.y.z"}))
out_servers = Array(Server).from_json(%([{"host":"a.b.c"}, {"host":"d.e.f"}]))

SaveMonitor.create!(input_servers: in_server, output_servers: out_servers)

m = MonitorQuery.new.first
m.output_servers.first.host == "a.b.c"