Make path joining robust to OS filesystems and remote URIs throughout code base

[mcember]

Currently, the code base uses a mix of os.path.join (mostly in unit tests) and posixpath.join

Os path join uses the OS defined delimiter, which isn't going to work for e.g. an S3 URI on windows. posixpath join uses the polix delimter which will work for non-filesystem URIs like S3 but not work for windows filesystems.

We have not detected these issues because our developers are not using windows, and the tests running with os.path.join are for local filesystem and not remote filesystem

Acceptance criteria

• Make custom path join utility which is aware of the scheme of a URI to understand if it is local filesystem or some other filesystem like s3:// or gs://
• Use this path join utility throughout code base
• Nice to have - ensure unit and integration tests run on Windows and S3