Adds in Array enums.

[jwoertink]

Fixes #940

This PR adds the ability to set an array column of enums. It also adds in the ability to specify a custom column converter if you're working with custom types that aren't built in to Avram.

class Post < BaseModel
  enum Reaction
    Like
    Heart
    Shocked
    Unlike
  end

  table do
    column reactions : Array(Post::Reaction) = [] of Post::Reaction, converter: PG::EnumArrayConverter(Post::Reaction)
  end
end

[jwoertink]

Originally I was going to use

{% elsif data_type.resolve < Enum %}

This would let us pick up the type and set the converter automatically; however, once I did this, it starts to resolve ALL the types. This means that in the comment model where we have belongs_to post : Post, the Post is undefined because it's resolved before crystal requires the post.cr file. It would end up being a breaking change since any model that references another later in alphabetical order would now have to be required in that file. (or you'd just require each model one by one in the proper order)

[jwoertink]

I actually don't know why this needs to be String.... but if you look at the method below, it's also converting the enum value to a string. Maybe a PQ thing? 🤔