cql3: values: add an internals-independent API to raw_value_view
cql3: values: add an internals-independent API to raw_value_view

Currently, raw_value_view is backed by a fragmented_temporary_buffer::view,
and many users of this type use it by extracting that internal representation.
However, we want to change raw_value_view so that it can be created both
from fragmented_temporary_buffer and from managed_bytes, so that we can switch
the internals of raw_value from bytes to managed_bytes. To do that we need
to prepare all users for that more general representation.

This commit adds an API which allow using raw_value_view without accessing its
internal representation. In the next commits of this series we will switch all
callers who currently depend on that representation to the new API,
and then we will remove the old accessors and change the internals.

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