Add functional tests for configured endpoint url resolution

The tests use a data file to load tests that enumerate the ways that the
endpoint URL can be defined and asserts that the correct endpoint is
used in a request and that it is ignored correctly if the appropriate
shared config file property or environment variable is supplied.

They also assert that the correct config section name and environment
variable name are used to configure and override the endpoint URL for
every AWS service.

tests/functional/configured_endpoint_urls/__init__.py

@@ -0,0 +1,12 @@
+# Copyright 2023 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"). You
+# may not use this file except in compliance with the License. A copy of
+# the License is located at
+#
+# http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file is
+# distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
+# ANY KIND, either express or implied. See the License for the specific
+# language governing permissions and limitations under the License.