Archive bandwidth files in relaydescs module.

Open questions:
 - Are we certain about descriptor annotation @type bandwidth-file
   1.0? This is consistent with respect to other relay descriptors,
   but it's not really intuitive with bandwidth file versions 1.0.0,
   1.1.0, etc.
 - Do we need config options to turn this off?

Temporary commit; necessary changes:
 - Change metrics-lib 2.6.0-dev dependency to 2.6.1 as soon as that's
   released.

Implements #30218.