Add brotli support

Brotli outperforms lzma and has a good pure-rust implementation; we should consider adding support for it to tor-dirclient, and to our eventual arti dircache implementation.

(C tor doesn't support brotli, but adding it wouldn't take more than a couple of days, if we think it's still relevant when we do this.)

One hard part would be picking the tuning parameters: at maximum compression, brotli is even more expensive than lzma; at high compression, it outperforms lzma without being dog-slow; but at lower levels it's not a 100% unreasonable streaming choice (thought zstd supposed to be faster for a given degree of compression).

Also see tor#33086 (closed) .