Today, when listing thread pools via the cat thread pool API, thread
pools are listed in a column-delimited format. This is unfriendly to
command-line tools, and inconsistent with other cat APIs. Instead,
thread pools should be listed in a row-delimited format.
Additionally, the cat thread pool API is limited to a fixed list of
thread pools that excludes certain built-in thread pools as well as all
custom thread pools. These thread pools should be available via the cat
thread pool API.
This commit improves the cat thread pool API by listing all thread pools
(built-in or custom), and by listing them in a row-delimited
format. Finally, for each node, the output thread pools are sorted by
thread pool name.
Relates #19721
533412e36f