Rename arguments and local variables in get_sstables_for_tablet to avoid
references to SSTable-specific terminology. This makes the function more
generic and better suited for reuse with different range types.
Generalize get_sstables_for_tablet by templating the return type so it
produces vectors matching the input range’s value type. This makes the
function more flexible and prepares it for reuse in tablet‑aware
restore.
Add getters for the first and last tokens in get_sstables_for_tablet to
make the function more generic and suitable for future use in the
tablet-aware restore code.
Move the download_sstable and get_sstables_for_tablet static functions
from sstables_loader into a new file to make them reusable by the
tablet-aware restore code.
