Add db.schema framework

jw.db.schema is a set of classes meant as an interface to describe a database schema.

Signed-off-by: Jan Lindemann <jan@janware.com>

tools/python/jwutils/db/schema/SingleForeignKey.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+
+from typing import Optional, List, Any
+
+from .ColumnSet import ColumnSet
+
+class SingleForeignKey:
+
+    def __init__(self, child_col, parent_col):
+        self.__child_col = child_col
+        self.__parent_col = parent_col
+
+    @property
+    def child_col(self):
+        return self.__child_col
+
+    @property
+    def parent_col(self):
+        return self.__parent_col