Right now, the function allows for passing the path to a file as a seastar::sstring,
which is then converted to std::filesystem::path -- implicitly to the caller.
However, the function performs I/O, and there is no reason to accept any other type
than std::filesystem::path, especially because the conversion is straightforward.
Callers can perform it on their own.
This commit introduces the more constrained API.
Closes#15266
c7fe5d7f94