Expose `on_delete` and `on_update` for ForeignKey fields

[ElijahAhianyo]

We currently have these policies defined

cot/cot/src/db/relations.rs

Lines 143 to 154 in 4620c6a

	#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Default)]
	pub enum ForeignKeyOnDeletePolicy {
	/// Don't do any additional actions on delete.
	NoAction,
	/// Prevent the delete if this model instance is referenced by another row.
	#[default]
	Restrict,
	/// Remove the referencing row if the referenced row is deleted.
	Cascade,
	/// Set the foreign key in the referencing row to [`None`].
	SetNone,
	}

and

cot/cot/src/db/relations.rs

Lines 175 to 187 in 4620c6a

	#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Default)]
	pub enum ForeignKeyOnUpdatePolicy {
	/// Don't do any additional actions on update.
	NoAction,
	/// Prevent the update if this model instance is referenced by another row.
	Restrict,
	/// Update the foreign key in the referencing row if the referenced row is
	/// updated.
	#[default]
	Cascade,
	/// Set the foreign key in the referencing row to [`None`].
	SetNone,
	}

However, we need to be able to expose them. Perhaps to use them like this:

#[model]
struct Foo {
   #[model(foreign_key(on_delete="cascade", on_update="set_none"))]
   bar: ForeignKey<Bar>
}