Add a switch to disable spider group by handler

This serves as a workaround for many bugs caused by the spider group
by handler (GBH) (labeled with spider-gbh). There is a system
variable spider_use_handler that could disable the group by
handler if it is set to 1*, but it was deprecated, and then deleted
in 10.10+ (MDEV-27256). So we could add
spider_disable_group_by_handler to 10.10+.

For context, see a comment in MDEV-26247[1]:

Here's a little demo showing disabling the spider GBH "fixes" the
issue. It could be a fallback / workaround, and it would be
interesting to see which other gbh bugs can also be mitigated by
this workaround.

637c1a3c315 upstream/bb-11.0-ycp-mdev-26247-disable-gbh MDEV-26247 [demo] disabling the spider gbh fixes the issue

update on [2023-09-22 Fri]: I confirm that all other bugs current
labeled with spider-gbh can be mitigated by this workaround:

• MDEV-31645
• MDEV-30392
• MDEV-29163
• MDEV-28998

So it makes sense to add a system variable e.g.
spider_disable_group_by_handler.

[1] https://jira.mariadb.org/browse/MDEV-26247?focusedCommentId=269982&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-269982

*: When spider_use_handler is set to 1, it skips creating
conn_holder on spider fields which causes the return of a NULL
pointer in spider_create_group_by_hander().