Instead of deep copying, use a shallow copy of the config value store
when using it for creating a new client from a session. Only use a deep
copy for updating the section provider.

Update behavior of the smart defaults update functions to use a copy
of the existing config providers in all cases. Update the logic for
determining which new provider to create.

Move unit tests for config provider smart defaults to functional tests
and rework them to better test behavior. A mocked defaults data file is
used to confirm that the correct values are loaded to protect against
changes to the live data file.

Adds a config provider to resolve the endpoint URL provided in an
environment variable or shared configuration file. It resolves the
endpoint in the following manner:

1. The value provided through the `endpoint_url` parameter provided to
   the client constructor.
2. The value provided by a service-specific environment variable.
3. The value provided by the global endpoint environment variable
   (`AWS_ENDPOINT_URL`).
4. The value provided by a service-specific parameter from a services
   definition section in the shared configuration file.
5. The value provided by the global parameter from a services definition
   section in the shared configuration file.
6. The value resolved when no configured endpoint URL is provided.

The endpoint config provider uses the client name (name used to
instantiate a client object) for construction and add to the config
value store. This uses multiple lookups to handle service name changes
for backwards compatibility.

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.

Add a debug statement when specifically ignoring the configured
endpoint URL from the shared configuration file or an environment
variable. This will not be logged if the endpoint URL is set through
the client constructor.

Simplify docstring and fix typos.

Add tests to check that setting the client creation parameter to ignore
the configured endpoint URLs is respected.