k/s: add a deterministic subscription-metadata hash

A group's assignment depends on the metadata of the topics it subscribes
to. The group stores a hash of that metadata, and a fresh hash that
differs from the stored one bumps the group epoch.

The algorithm is Kafka's KIP-1101: one hash per topic, and one hash for
the group over the per-topic hashes in order of ascending topic name.
The value stays inside one Redpanda cluster, so the byte encoding is
this module's own: fixed-width big-endian integers, and a length before
every string.

Signed-off-by: Oren Leiman <oren.leiman@redpanda.com>