When fetching onion service descriptors, use minimal and uniform HTTP headers.

When we're fetching onion service descriptors, we should only advertise the compression algorithms that all clients have. (I think that's deflate, identity.) Otherwise, we create a fingerprinting opportunity where different profiles of arti can be distinguished by the HsDirs, which is not desirable.

We should also look over our HTTP headers for any other unnecessary headers sent here.

cc @gabi-250