Allow replacing build IDs with digests

[dmartin]

Adds a new optional command-line flag, -s / --salt, which takes a string value.
When specified, numeric build IDs in artifact download URLs (and the on-disk cache) are replaced by the hexadecimal SHA-256 digest of {build_id}:{salt}.

For example, instead of:

http://localhost:4201/1/file.txt
http://localhost:4201/2/file.txt

you might have:

http://localhost:4201/6dc430e5b8a7a1c59589d48d6cd84e5b11ae6cbf0b0733164b5acc59a21a2734/file.txt
http://localhost:4201/51bc37a442bc79a1018d3097c5e1c2c6fd7efde9c8ea611c671f4833961ebb61/file.txt

This prevents players from enumerating artifact download URLs across builds of a challenge.

Note that clients must also apply this same transformation if generating URLs based on build IDs.