cql3: make add_restriction helper functions public
cql3: make add_restriction helper functions public

In order to allow building statement restrictions manually
instead of providing WHERE clause from CQL layer, helper functions
that add single restrictions are made public.
Message-Id: <f22d41f29a9cd674efeac7b0f9683d2e7babdbf4@scylladb.com>

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