Support HTTP basic auth

[mrled]

We could extend our viewer-request function to implement HTTP Basic Authentication.

Use cases:

• Staging sites
• Draft posts to share with proofreaders in advance
• ... etc

Netlify does this, but it requires pro or enterprise.

I think this wouldn't make the KVS design any worse. There's no real reason to support auth AND a redirect for the same path, so if the KVS value looks like a path, it's a redirect; if it looks like something else, it's auth. We could also move to prefixes, like a:USERNAME:PASSWORD or something like that, which would be more extensible for future features.