gh-35271: add .conductor() and .order_of_conductor() methods for orders in quadratic fields
This patch adds two tiny convenience methods:
* `.conductor()` for orders in quadratic fields, returning the unique
positive integer $f$ such that the discriminant equals $f^2\cdot D$ with
$D$ a fundamental discriminant.
* `.order_of_conductor()` for quadratic fields, returning the unique
order with the given conductor.
URL: https://github.com/sagemath/sage/pull/35271
Reported by: Lorenz Panny
Reviewer(s): Travis Scrimshaw
