Add database validation rules exists_in_db and unique_in_db

[girardinsamuel]

Usage

validate.exists_in_db("email", "users")
validate.exists_in_db("name", "users", "first_name")
validate.exists_in_db("email", "app.models.User")
validate.exists_in_db("email", "app.models.User", connection="sqlite")

validate.unique_in_db("email", "users")
validate.unique_in_db("my_unique_email", "app.User", "email")
validate.unique_in_db("email", "app.models.User")

Fix #586