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.

[ndrscodes]

could we please get this merged? it really is a missing feature right now. @raema do you plan on fixing the failing pipeline?

[Lzzzzzt]

@raema hello, there are some CI errors, could you help please fix them? Thanks a lot.

[ndrscodes]

@Lzzzzzt could I open another PR and copy over @raema's changes to this while fixing the issues if @raema does not respond?

[Lzzzzzt]

@Lzzzzzt could I open another PR and copy over @raema's changes to this while fixing the issues if @raema does not respond?

sure