Allow pageinspect's bt_page_stats function to return a set of rows instead of a single row

Hello Hackers,

While working on one of my blogs on the B-Tree indexes
<https://www.percona.com/blog/postgresql-14-b-tree-index-reduced-bloat-with-bottom-up-deletion/>,
I needed to look at a range of B-Tree page statistics. So the goto solution
was to use pageinspect. However, reviewing stats for multiple pages meant
issuing multiple queries. I felt that there's an opportunity for
improvement in the extension by extending the API to output the statistics
for multiple pages with a single query.

That attached patch is based on the master branch. It makes the following
changes to the pageinspect contrib module:
- Updates bt_page_stats_internal function to accept 3 arguments instead of
2.
- The function now uses SRF macros to return a set rather than a single
row. The function call now requires specifying column names.

The extension version is bumped to 1.11 (PAGEINSPECT_V1_11).
To maintain backward compatibility, for versions below 1.11, the multi-call
mechanism is ended to keep the old behavior consistent.

Regression test cases for the module are updated as well as part of this
change. Here is a subset of queries that are added to the btree.sql test
case file for pageinspect.

----
CREATE TABLE test2 AS (SELECT generate_series(1, 5000) AS col1);
CREATE INDEX test2_col1_idx ON test2(col1);
SELECT * FROM bt_page_stats('test2_col1_idx', 1, 2);
SELECT * FROM bt_page_stats('test2_col1_idx', 1, 0);
SELECT * FROM bt_page_stats('test2_col1_idx', 0, 1);
SELECT * FROM bt_page_stats('test2_col1_idx', 1, -1);
DROP TABLE test2;
----

Regards.

--
Hamid Akhtar,
Percona LLC,
URL : www.percona.com
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