directory_lister provides a simpler interface compared to lister.
After creating the directory_lister,
its async get() method should be called repeatedly,
returning a std::optional<directory_entry> each call,
until it returns a disengaged entry or an error.
This is especially suitable for coroutines
as demonstrated in the unit tests that were added.
For example:
```c++
auto dl = directory_lister(path);
while (auto de = co_await dl.get()) {
co_await process(*de);
}
```
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#9835
* github.com:scylladb/scylla:
sstable_directory: process_sstable_dir: use directory_lister
sstable_directory: process_sstable_dir: fixup indentation
sstable_directory: coroutinize process_sstable_dir
lister: add directory_lister
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