Add ENCODING option to COPY TO/FROM and file_fdw.

Add ENCODING option to COPY TO/FROM and file_fdw.
File encodings can be specified separately from client encoding.
If not specified, client encoding is used for backward compatibility.

Cases when the encoding doesn't match client encoding are slower
than matched cases because we don't have conversion procs for other
encodings. Performance improvement would be be a future work.

Original patch by Hitoshi Harada, and modified by me.