Add `.table` method to models

[vladfaust]

This is tricky, because table is defined in class-level annotation like this:

@[Onyx::SQL::Model::Options(table: "users")]
class User
end

One option is to make Model.schema macro define .table class method.

[vladfaust]

This works, though :sweat_smile:

class User
  def table
    "users"
  end
end

user = uninitialized User
pp user.table