pgsql: Move code specific to pg_ndistinct to new file

Move code specific to pg_ndistinct to new file

This new file is named pg_ndistinct.c and includes all the code directly
related to the data type pg_ndistinct, extracted from the extended
statistics code.

Some patches are under discussion to change its input and output
functions, and this separation makes the follow-up changes cleaner by
separating the logic related to the data type and the multivariate
ndistinct coefficient core logic in mvdistinct.c.

Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Co-authored-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/aQ2k8--a0FfwSwX9@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a5523123430f39ba26a0d0250fbe10a396ab0703

Modified Files
--------------
src/backend/statistics/mvdistinct.c | 85 -----------------------------
src/backend/utils/adt/Makefile | 1 +
src/backend/utils/adt/meson.build | 1 +
src/backend/utils/adt/pg_ndistinct.c | 102 +++++++++++++++++++++++++++++++++++
4 files changed, 104 insertions(+), 85 deletions(-)