feat: one-to-many relation helper

Allow one to specify that a field of a model is a foreign key.
It will generate a bunch of helper methods to query related entities
from one entity.

DRAFT.md

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+# Design draft
+
+This document is at the attention of the developers of sqlxgentools.
+
+## Implementing basic relation ship
+
+### Issues
+
+Problems with Struct non-flexibility
+
+### Turning the problem around: Views
+
+### hasMany / belongsTo relationship
+
+So we can implements a method
+
+```rs
+use repositories::impls::post::RelationShips;
+
+let post = PostRepository::new(db).get_one_by_id("id_machin")?;
+post.first_name // OK
+let authors = post.get_authors()? // we need to require the implementation
+
+```
+
+.relations() => give you a RelationFetcherBuilder
+.of(entity) => give you a RelationFetcher
+.author
+
+```rs
+
+let post_repo = PostRepository::new(db);
+let author: User = post_repo.relations()
+    .of(post)
+    .author().ok_or(Err)?;
+let comments: Vec<Comment> = post_repo.relations()
+    .of(post)
+    .comments().ok_or(Err)?;
+```