tor-hscrypto: Implement FromStr for HsClientDescEncKey.
We will use this impl for parsing the authorized clients of a hidden service running in "restricted discovery" mode.
NB: "Restricted discovery" is the new terminology for "services that require client authorization", i.e. services that implement "client authorization" as described in rend-spec HS-DESC-ENC. Note we haven't fully transitioned to the new terminology yet (see #1476 (closed)).
Part of #1292 (closed)