Add a DEFAULT option to COPY FROM

Add a DEFAULT option to COPY  FROM

This allows for a string which if an input field matches causes the
column's default value to be inserted. The advantage of this is that
the default can be inserted in some rows and not others, for which
non-default data is available.

The file_fdw extension is also modified to take allow use of this
option.

Israel Barth Rubio

Discussion: https://postgr.es/m/CAO_rXXAcqesk6DsvioOZ5zmeEmpUN5ktZf-9=9yu+DTr0Xr8Uw@mail.gmail.com