test_custom_types: Test module with fancy custom data types

test_custom_types: Test module with fancy custom data types

This commit adds a new test module called "test_custom_types", that can
be used to stress code paths related to custom data type
implementations.

Currently, this is used as a test suite to validate the set of fixes
done in 3b7a6fa15720, that requires some typanalyze callbacks that can
force very specific backend behaviors, as of:
- typanalyze callback that returns "false" as status, to mark a failure
in computing statistics.
- typanalyze callback that returns "true" but let's the backend know
that no interesting stats could be computed, with stats_valid set to
"false".

This could be extended more in the future if more problems are found.
For simplicity, the module uses a fake int4 data type, that requires a
btree operator class to be usable with extended statistics.  The type is
created by the extension, and its properties are altered in the test.

Like 3b7a6fa15720, this module is backpatched down to v14, for coverage
purposes.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aaDrJsE1I5mrE-QF@paquier.xyz
Backpatch-through: 14