Add pg_stat_kind_info system view

Hey hackers,

In 7949d959458[0], we introduced pluggable cumulative statistics. 
Extensions could now register their own custom statistics. However, 
there was no way for an extension author to introspect their custom 
statistics through SQL, and I don't believe there is currently a way to 
introspect builtin statistics either.

Additionally, there was no way to know how much shared memory each 
statistics kind was using. We could only know the total shared memory 
used by the statistics subsystem.

	dbltap@postgres=# SELECT * FROM pg_shmem_allocations WHERE name = 'Shared Memory Stats';
	        name         |    off    |  size  | allocated_size
	---------------------+-----------+--------+----------------
	 Shared Memory Stats | 153456640 | 321976 |         321976

In this patch, I have added a new system view: pg_stat_kind_info built 
on a new function pg_stat_get_kind_info(). The view has the following 
columns:

- id: id of the statistics kind
- name: name of the statistics kind
- count: number of entries for the statistics kind
- builtin: whether the statistics kind of builtin or not
- shared_size: shared memory size of each entry

	dbltap@postgres=# SELECT * FROM pg_stat_kind_info;
	 id |          name           | count  | builtin | shared_size
	----+-------------------------+--------+---------+-------------
	  1 | database                | (null) | t       |         288
	  2 | relation                | (null) | t       |         248
	  3 | function                | (null) | t       |          56
	  4 | replslot                | (null) | t       |         120
	  5 | subscription            | (null) | t       |         120
	  6 | backend                 | (null) | t       |        2952
	  7 | archiver                |      1 | t       |           0
	  8 | bgwriter                |      1 | t       |           0
	  9 | checkpointer            |      1 | t       |           0
	 10 | io                      |      1 | t       |           0
	 11 | lock                    |      1 | t       |           0
	 12 | slru                    |      1 | t       |           0
	 13 | wal                     |      1 | t       |           0
	 25 | test_custom_var_stats   |      0 | f       |          40
	 26 | test_custom_fixed_stats |      1 | f       |          56
	(15 rows)

I am not sure that shared_size is a good column name, and I needed to 
include pgstat_internal.h in pgstatfuncs.c to get everything working. 
I'm also curious to hear if anyone thinks there is other valuable 
information to expose.

[0]: https://github.com/postgres/postgres/commit/7949d9594582ab49dee221e1db1aa5401ace49d4

-- 
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)