Set custom User-Agent string for hstspreload

[devonobrien]

The outbound scans used by hstspreload.org as well as bulk updates to check for preloading eligibility have started to be blocked by several CDNs' spam/fraud detection, which is preventing the addition and removal of domains in several known instances (and likely more we don't know about). Adding a custom user agent string lets these agents be allowlisted with a (User-Agent, ASN)-tuple.

I suggest "hsts-preload-bot" as a possible UA string, but am welcome to other suggestions.

[nharper]

There's some discussion of this on #107.

[devonobrien]

Thanks! I didn't see any active issues with this, but probably should have dug deeper into closed issues. Regarding your comments on #107, I'm also not generally a fan of encouraging allowlisting by user agent string, but this is the only mechanism we've been presented with to circumvent the automated blocking of domain scans. I'll re-open #107 and continue the discussion there.