orm: allow use of mut db orm.Connection (#17439)

vlib/orm/orm_mut_db_test.v

@@ -0,0 +1,36 @@
+import db.sqlite
+
+struct User {
+	id   int    [primary; sql: serial]
+	name string
+}
+
+fn get_users(mut db sqlite.DB) []User {
+	return sql db {
+		select from User
+	}
+}
+
+fn test_orm_mut_db() {
+	mut db := sqlite.connect(':memory:') or { panic(err) }
+
+	sql db {
+		create table User
+	}
+
+	first_user := User{
+		name: 'first'
+	}
+	second_user := User{
+		name: 'second'
+	}
+
+	sql db {
+		insert first_user into User
+		insert second_user into User
+	}
+
+	users := get_users(mut db)
+
+	assert users.len == 2
+}