[datlechin]

Summary

• db.c.aggregate([...]).itcount() in the SQL editor returns the number of documents the pipeline produces, counted on the server with a $group stage, the way find(...).count() already is. feat(linux): connect to MongoDB, browse its collections and run shell statements #3820 refused it as an unsupported cursor method.
• A pipeline that ends in $out or $merge is still refused with itcount(): the server takes no stage after one that writes.
• explain, whether as .explain() or the {explain: true} option, still asks the planner instead.

Test plan

• New unit test an_aggregates_itcount_counts_on_the_server: the plan ends in the $group stage and is counted; $merge is refused; {explain: true} still gives an explain.
• a_find_reads_the_fields_it_projects_in_the_order_it_sorts (requires docker) now also runs aggregate([{$match: ...}]).itcount() against MongoDB and reads 2.
• clippy -p tablepro-driver-mongodb --all-targets -D warnings, cargo fmt --check.

Refs #3646