Support nested configuration

Problem/Motivation

Configuration values that are nested are not currently supported.

Steps to reproduce

1. Create a controller and route.
2. Define nested configuration, like my_module.settings.foo.bar.
3. Set the _config requirement for the route to use that nested configuration.

The module will parse the config object name as my_module.settings.foo and the config key as bar.

Proposed resolution

Parse configuration names properly to allow nested values to be used.

Remaining tasks

• Change the config name parsing in ConfigRouteSubscriber.
• Make sure the current test cases pass with the new parsing.
• Add test cases for nested configuration.
• Make it clear in the user guide that nested configuration is supported.
• Update the change log.