feat: implement timestamp callback plugin to show simple timestamp for each header (#8308)

* feat: add community.general.timestamp callback plugin

* feat: add minimal integration tests for timestamp callback plugin

* feat: add maintainers for timestamp callback plugin

* fix: correct license

* fix: remove type annotation for the older python environment

* fix: remove unnecessary comment

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

* fix: add trailing period

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

* fix: split long description into list

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

* fix: remove default and add type

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

* fix; add type

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

* fix: split long description into list

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

* fix: improve description for format_string to describe usable format codes

* fix: clarify the original codes and add copyright from that

* fix: shorten long lines

* fix: correct link format

* fix: add seealso section

* fix: add ignore entries for EOL CI

* fix: update seealso to correctly associate with related plugin

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

---------

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

tests/integration/targets/callback_timestamp/aliases

@@ -0,0 +1,6 @@
+# Copyright (c) 2024, kurokobo <kurokobo@protonmail.com>
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or <https://www.gnu.org/licenses/gpl-3.0.txt>)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+azp/posix/1
+needs/target/callback