Flatten CloudSearch commands arguments
Code review feedback

* Updated documentation
* FlattenCommands -> FlattenArguments
* modify_args -> flatten_args, _find_nested_arg, _merge_member_config
* service -> service_name
* CLI registration is separated from object instantiation now
* AssertIn for tests
* Added a test for list types
* Added type to FlattenedArgument
* Fix CloudSearch argument hydrating to use correct types
* Some refactoring
Add CloudSearch tests
Move CloudSearch out of preview
Update changelog
Merge pull request #730 from danielgtaylor/cs-flatten

Move CloudSearch out of preview. Fixes #730.

Adds customizations to make using CloudSearch easier from the commandline.