Return `std::filesystem::path` from `TempDir::path()` and `TempDir::file()`

[mvadari]

Follow-up from a review comment on #7012: #7012 (comment)

In include/xrpl/basics/FileUtilities.h, TempDir::path() and TempDir::file() both return std::string:

[[nodiscard]] std::string
path() const;

[[nodiscard]] std::string
file(std::string const& name) const;

Now that the codebase uses std::filesystem (#7012), these should probably return std::filesystem::path instead, so callers don't have to round-trip through strings. file() could likewise take a std::filesystem::path const& parameter.

This was intentionally left as-is in #7012 to keep that PR focused on the boost::filesystem → std::filesystem swap.

Work

• Change the return types (and file()'s parameter type) to std::filesystem::path.
• Update call sites, which are mostly in tests/benchmarks.