pgsql: Move information about pgstats kinds into its own header pgstat_

Move information about pgstats kinds into its own header pgstat_kind.h

This includes all the definitions for the various PGSTAT_KIND_* values,
the range allowed for custom stats kinds and some macros related all
that.

One use-case behind this split is the possibility to use this
information for frontend tools, without having to rely on pgstat.h and a
backend footprint.

Author: Michael Paquier
Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Z24fyb3ipXKR38oS@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d35ea27e51c05cbe3575d50a6b99d64f20a3a742

Modified Files
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src/include/pgstat.h | 57 +-------------------------------
src/include/utils/pgstat_kind.h | 72 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 73 insertions(+), 56 deletions(-)