Provider implementations will live is subpackages, to avoid pulling in
heavy cloud sdk dependencies when the user may only want to use one
backend

Co-authored-by: GitLab Duo <gitlab-duo@gitlab.com>

Initial multi-backend objectstorage client implementation

See merge request !546

Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Luke Hollinda <lhollinda@gitlab.com>