Support password protected demos on Cloud Run

[stevejalim]

At the moment, to support password-protected demos, we need to use our Heroku instances, because:

• developers have to set BASIC_AUTH_CREDS="username:password" as an env var, but because those need to be committed to get them only Cloud Run, it's not viable to commit them to the public repo
• Secrets are shared across all Cloud Run demo services, so if set BASIC_AUTH_CREDS in Secrets, they'd apply to all demos

So, obvs, we need a solution - and one that ideally is very light touch

[stevejalim]

One option might be to stably generate random-but-readable passwords using the branch name as a seed then log them out to the private logs for a developer to pick up and share as needed. The credentials would be different per branch name, but not change with each commit to that branch