Adding fetch_upload method to client

[Ponni-M]

https://github.com/zulip/python-zulip-api/issues/628

[timabbott]

@Ponni-M can you restructure the commits to first refactor the /api/ prefix stuff, with a commit message explaining why that's correct, and then one adding the new endpoint?

Check out the Zulip commit message guidelines for more details: https://zulip.readthedocs.io/en/latest/contributing/version-control.html#commit-messages

[LoopThrough-i-j]

@Ponni-M thanks for working on this, added a comment. Everything else LGTM.

[zulipbot]

Heads up @Ponni-M, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.