[PR #8456/6f8f12f7 backport][stable-9] Feature filter keep_keys (#8462)

Feature filter keep_keys (#8456)

* Add filter keep_keys. Implement feature request #8438

* Fix comment indentation.

* Fix regex reference.

* Fix indentation.

* Fix isinstance list.

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/plugin_utils/keys_filter.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update plugins/filter/keep_keys.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update documentation, examples, and integration tests.

* _keys_filter_target_str returns tuple of unique target strings if
target is list. Update documentation, function comments, and error
messages.

* Sort maintainers.

* Update plugins/filter/keep_keys.py

Co-authored-by: Felix Fontein <felix@fontein.de>

* Update examples with explicit collection.

---------

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 6f8f12f762)

Co-authored-by: Vladimir Botka <vbotka@gmail.com>

.github/BOTMETA.yml

@@ -157,6 +157,8 @@ files:
   $filters/jc.py:
     maintainers: kellyjonbrazil
   $filters/json_query.py: {}
+  $filters/keep_keys.py:
+    maintainers: vbotka
   $filters/lists.py:
     maintainers: cfiehe
   $filters/lists_difference.yml:
@@ -1417,6 +1419,8 @@ files:
     ignore: matze
     labels: zypper
     maintainers: $team_suse
+  $plugin_utils/keys_filter.py:
+    maintainers: vbotka
   $plugin_utils/unsafe.py:
     maintainers: felixfontein
   $tests/a_module.py: