Authorities should put a hash of the bandwidth file in their votes

After #3723 (moved) is completed, authorities will list the bandwidth file headers in their votes. After #21377 (moved) is completed, they will make bandwidth files available at a standard URL.

But if we want to cryptographically verify the exact bandwidth file that was used in a vote, we need to put a hash of the file in the vote. Let's do it in an extensible way:

bandwidth-file hash-algo base64(hash(bandwidth-file-content))