Extended Statistics set/restore/clear functions.

This is a separate thread for work started in [1] but focused purely on
getting the following functions working:

* pg_set_extended_stats
* pg_clear_extended_stats
* pg_restore_extended_stats

These functions are analogous to their relation/attribute counterparts, use
the same calling conventions, and build upon the same basic infrastructure.

I think it is important that we get these implemented because they close
the gap that was left in terms of the ability to modify existing statistics
and to round out the work being done to carry over statistics via
dump/restore and pg_upgrade i [1].

The purpose of each patch is as follows (adapted from previous thread):

0001 - This makes the input function for pg_ndistinct functional.

0002 - This makes the input function for pg_dependencies functional.

0003 - Makes several static functions in attribute_stats.c public for use
by extended stats. One of those is get_stat_attr_type(), which in the last
patchset was modified to take an attribute name rather than attnum, thus
saving a syscache lookup. However, extended stats identifies attributes by
attnum not name, so that optimization had to be set aside, at least
temporarily.

0004 - These implement the functions pg_set_extended_stats(),
pg_clear_extended_stats(), and pg_restore_extended_stats() and behave like
their relation/attribute equivalents. If we can get these committed and
used by pg_dump, then we don't have to debate how to handle post-upgrade
steps for users who happen to have extended stats vs the approximately
99.75% of users who do not have extended stats.

This patchset does not presently include any work to integrate these
functions into pg_dump, but may do so once that work is settled, or it may
become its own thread.

[1]
https://www.postgresql.org/message-id/CADkLM=c6NHdXU+d+m1yASZ4NT_qye1LCnaU2Vgf8YJ80jJT-Qg@mail.gmail.com