Add ability to define custom data types

[MrKirat]

Hi!

Currently, the library supports only a limited number of types - array, hash, big_decimal, integer, float, string, boolean.
I have a need to save custom types into a DB, so I decided to try to add the functionality that will allow me to do that. During implementation, I also tried to refactor a little bit of other code in order to make it more readable.

So here is what I've done so far:

• Added ability to define custom types with their own serializers by splitting fields by types into separate classes (https://github.com/MrKirat/rails-settings-cached/tree/kirat/add-ability-to-add-custom-types/lib/rails-settings/fields) and adding dynamic lookup (https://github.com/MrKirat/rails-settings-cached/blob/kirat/add-ability-to-add-custom-types/lib/rails-settings/fields/base.rb#L62-L71)
• Moved the logic of methods defining into a separate class (https://github.com/MrKirat/rails-settings-cached/blob/kirat/add-ability-to-add-custom-types/lib/rails-settings/methods_defining.rb)
• Moved the logic of the cache into a separate module (https://github.com/MrKirat/rails-settings-cached/blob/kirat/add-ability-to-add-custom-types/lib/rails-settings/cache.rb)
• Fixed some Rubocop issues

Take note, that I was trying to keep us much backward compatibility as I can. At this moment it seems like all tests are being passed successfully.

Maintainers (@codez @berkos @huacnlee), could you please let me know whether it makes sense?

If yes, I will prepare the PR for the merge. If not - you are free to close it :)

[huacnlee]

Too many changes, please do not change code style.

[MrKirat]

@huacnlee I've pushed new changes only focused on the ability to add custom data types. Could you please check the changes again?

[huacnlee]

Great work!