Cloudwanderer

- Implemented `mypy` type checking
- Corrected a huge number of type hints
- Reformalised arguments for `get_resources` on storage_connectors.
- Fixed DynamoDB pagination

- Added `parent_urn` and `subresource_urn` properties to the `CloudWanderResource` class.
- Added new DynamoDB secondary index `parent_urn`
- Fixed bug where subresources were not cleaned up when `write_resource` was called on `CloudWanderer`
- Stripped out DynamoDB record specific attributes which were polluting Secondary Attributes dictionaries.
- Changed `AWSInterface` `get_resources` to expect specific service, resource type, region arguments instead of reusing the arguments from CloudWanderer `write_resources`.
- Added `get_actions` to `AWSInterface` which returns a list of `GetAndCleanUp` objects which pair `GetAction`s and `CleanUpAction`s.
- CloudWanderer's `write_resources` now contains the logic for iterating over each `GetAction`, calling `get_resources` on `AWSInterface` and calling `delete_resource_of_type_in_account_region` on each StorageConnector in accordance with the `CleanUpAction`.
- Added `get_and_cleanup_actions` property to `CloudWandererBoto3Resource` so the responsibility for defining `GetAndCleanUp` objects resides with the resource. This provides maximum flexibility for asymmetric region/resource discovery (like S3 buckets).
- Added `get_empty_service` to `Boto3Services` to minimise the number of unnecessary (and expensive) Boto3 client creations when generating get and cleanup actions.

- Fixed bug causing subresources to inherit the secondary attributes of their parent resource erroneously.

- Fixed bug that prevented global services with regional resources being cleaned up properly.
- Fixed bug that prevented subresources from being cleaned up.
- Subresources are now written by `write_resource`.

- Added `get_resource` to `CloudWanderer` to allow the writing of a single resource based on its URN
- Added `get_resource` to `CloudWandererAWSInterface` to allow the getting of a single resource based on its URN
- Added `get_resource_by_urn` to `Boto3Getter` to support `CloudWandererAWSInterface`
- Normalised custom error exception names to have Error at the end as per PEP8
- Renamed `CloudWandererBoto3Interface` to `CloudWandererAWSInterface`
- Renamed `AwsUrn` to `URN`
- Added `storage_connector_generator` to `write_resources_concurrently` to handle non thread safe storage connectors (hopefully fixing 86)
- Refactored `Boto3Getter` into service and resource oriented wrappers for Boto3 objects to make it more domain drive and easier to understand.
- Added support for Boto3 subresources (where they match the CloudWanderer definition of a subresource)

- Changed the way `exclude_resources` works so you can differentiate between CloudFormation Stacks and OpsWorks Stacks 70
- Formalised the interface between the `CloudInterface` > `CloudWanderer` > `StorageConnector` by
converting Boto3 resources to `CloudWandererResources`
- Removed `write_secondary_attributes` from `BaseStorageConnector` as it's no longer required to be public.
- Added `name` argument to `get_secondary_attribute` allowing you to get secondary attributes by name.
- Ensured that all resource attributes that *can* exist *do* exist when `CloudWandererResource` returned from `CloudWandererBoto3Interface`, irrespective of whether they were returned in that particular API call.

New Resources

- apigateway rest_api
- secretsmanager secret