Adjust Return Value of get_host_subject_id in source_repo.py

Currently, [get_host_subject_id](https://github.com/biocore/microsetta-private-api/blob/master/microsetta_private_api/repo/source_repo.py#L193) returns a tuple, which is causing host_subject_id to be represented in Qiita as `['VALUE']` rather than `VALUE`. The function's value should be changed to a string and a unit test should be created to assert that it has been fixed.

We're intentionally deferring on this change until January 2024, as there are downstream consequences in Qiita and Redbiom that we'll need to deal with simultaneously.