alternator: better key type parsing
alternator: better key type parsing

The supported key types are just S(tring), B(lob), or N(umber).
Other types are valid for attributes, but not for keys, and should
not be accepted. And wrong types used should result in the appropriate
user-visible error.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla