Move the key_to_str() template function from a file-local static in
db/large_data_handler.cc to keys/keys.hh so it can be reused by:
- large_data_handler.cc for log messages
- virtual tables (db/virtual_tables.cc) for converting binary keys
to human-readable CQL display
- scylla-sstable for JSON output of LargeDataRecords
No functional change.
d4283d0ffc