fix: tb-gcp-uploader to show flags in "--help" correctly, and new flags for api uri overriding

[yfang1]

• Plumbing the flags from uploader_main to main such that "tb-gcp-uploader --help" will show flag definitions
• defining new flags for api and web server uri, to allow for communicating with non-prod APIs

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.