Feature request: Add std::filesystem::path overload for file_stdio::open

`file_stdio::open` currently only accepts a `char const* path`. Recently I had a std::filesystem::path and wanted to call `file_stdio::open`, noticed that I had to do `mypath.c_str()` to make it compile, briefly thought "hm that's a bit weird", but it worked and I didn't think further about it.

Then a week later, I noticed my code no longer compiled on Windows, because there, `mypath.c_str()` returns `wchar_t const*`(!). For the time being I made the compiler shut up by doing `mypath.string().c_str()`, but I'm not even sure that's correct, and really, it just made me wonder why Beast is doing this to me. :-)

Update: I see `file_stdio::open` specifically requires UTF-8, so maybe I should be using `reinterpret_cast(mypath.u8string().c_str())`? If so, that's not the most ergonomic API for opening a file specified by a std::filesystem::path.

Search for the declaration and implementation of file_stdio::open, then read how its current char const* path argument handles UTF-8. Check the surrounding tests or call sites for platform-specific path behavior. Done means a std::filesystem::path can be passed ergonomically and works correctly on Windows and other supported platforms.

Written by the indexing model from the issue text.