Externalize Attributes

[mvcds]

Allow hubi to use attribute types which are not in this same repository.

For instance, configuring a required string to have a max length of 30 digits in a lot of domain files is boring, so allowing users to create a AlphanumericWithFixedLengthTo30 just for that project would avoid a lot of repeatition.

Useful link: https://nodejs.org/en/blog/npm/peer-dependencies/