cql3::untyped_result_set: Allow "get_map" to be explicit about result type
cql3::untyped_result_set: Allow "get_map" to be explicit about result
type

Allow providing both hash/equal etc for resulting map, as well
as explicit data_types for the deserialization.
Also allow direct extraction of kv-pairs to iterator, for more advanced
unpacking.

scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla,duarten/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla