Automatically create relations

[steffendsommer]

Since we're already annotating relations, I think it makes sense to have Sourcery create relations. E.g. in the case of (inside Employee.swift):

// sourcery: preparation = foreignId, unique = false, foreignTable = Company
internal var companyId: Identifier?

Then it could create an extension on Employee with a property called company which would return the parent. Similarly it could create an extension on Company with a property called employees which would return the children.

We could consider if it makes sense to have an annotation to indicate 1-1 relations so instead of returning all the children it would return the first children.