feat: implement AsReadFd and AsWriteFd for File

[raema]

Problem

monoio::fs::File can't be used with splice traits (SpliceSource, SpliceSink) because it
doesn't implement AsReadFd or AsWriteFd.

Approach

Implement AsReadFd and AsWriteFd for File, matching the pattern used by TcpStream, UnixStream, and Pipe.

The implementation delegates to the file's internal SharedFd:

• as_reader_fd() -> wraps self.fd in SharedFdWrapper
• as_writer_fd() -> wraps self.fd in SharedFdWrapper

This enables file.splice_to_pipe() and file.splice_from_pipe() calls.