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.

docs/tutorials/quick_start.md

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+# Quick start with sqlxgentools
+
+Steps:
+- Install the crate
+- Declare your models
+- Generate migrations
+- Generate repositories
+- Use repositories in your code