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.huinker@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aQ2k8--a0FfwSwX9@paquier.xyz