Support/fix nested folders of source documents

Today I tried to use the CLI to create a labelling job from an S3 prefix with nested folders - for example:

```
s3://comprehend-semi-structured-docs-${AWS_REGION}-${AWS_ACCOUNT_ID}/
docs/
folder-1/
doc1.pdf
folder-2/
doc2.pdf
doc3.pdf
```

I created the job using the base folder as the input path, something like `python bin/comprehend-ssie-annotation-tool-cli.py --input-s3-path s3://{bucket}/docs/`

...But the job failed due to `NoSuchKey` errors in the pre-annotation Lambda function:

```
[ERROR] NoSuchKey: An error occurred (NoSuchKey) when calling the GetObject operation: The specified key does not exist.
Traceback (most recent call last):
File "/var/task/lambdas/pre_human_task_lambda.py", line 346, in lambda_handler
pdf_s3_resp = s3_client.get_object_response_from_s3(source_ref)
File "/var/task/utils/s3_helper.py", line 36, in get_object_response_from_s3
return self.s3_client.get_object(Bucket=bucket, Key=path)
File "/var/task/botocore/client.py", line 514, in _api_call
return self._make_api_call(operation_name, kwargs)
File "/var/task/botocore/client.py", line 938, in _make_api_call
raise error_class(parsed_response, operation_name)
```

Looking at the generated input manifest, it seems like the preparation hasn't correctly handled the nested folders because everything is showing as filenames directly under the `input-s3-path` - for example:

```
{"source-ref": "s3://{bucket}/docs/doc1.pdf", "page": "1", "metadata": {...}, "annotator-metadata": null, "primary-annotation-ref": null, "secondary-annotation-ref": null}
{"source-ref": "s3://{bucket}/docs/doc2.pdf", "page": "1", "metadata": {...}, "annotator-metadata": null, "primary-annotation-ref": null, "secondary-annotation-ref": null}
{"source-ref": "s3://{bucket}/docs/doc3.pdf", "page": "1", "metadata": {...}, "annotator-metadata": null, "primary-annotation-ref": null, "secondary-annotation-ref": null}
```

I didn't see anything in the README guide suggesting we ***can't*** have nested folders in the input prefix, so not quite sure whether this is a bug or a feature request?