23.3: when apt keyids provided in #cloud-config, implicit dependency on gpgconf breaks Ubuntu minimal images

cloud-init version: 23.3
ubuntu 23.10 (Mantic) minimal images do not contain gpg, gnupg or gpgconf packages by default

Upstream commit https://github.com/canonical/cloud-init/commit/842d0452 introduced a strict dependency on gpgconfig utility to kill any gpg-agent/dirmngr daemons spawned by a `gpg --recv-keys command`

Although cloud-init debian/control has a Recommends: gnupg, Recommends packages are not installed in Ubuntu minimal images, which leaves images without gpg or gpgconf. This leaves cloud-init.log with tracebacks like the following[1].

cloud-init package calling the utility gpgconf for any apt:sources configuration breaks daily Ubuntu minimal builds on Mantic builds and should be seen as an SRU blocker for cloud-init of 23.3.

This bug is only triggered in ubuntu minimal images (without gpgconf) are launched with apt:sources config. Any ubuntu-minimal image launches without "apt: sources:" in #cloud-config user-data will not trigger this issue.

Short-term solution:
 - To avoid pulling in unnecessary package dependencies and bloating images, cloud-init should approach this daemon cleanup using common system utilities (kill) or python modules (os.kill) if dirmgr/gpg-agent are present and active. In absence of said services/utilities, cloud-init should not attempt to kill any dirmngr/gpg-agent services.

Long-term solution:
 - cloud-init to install necessary gpg dependencies during first boot if the image doesn't have recommended utilities, and optional user-data requires gpg keyid interaction
 - Adapt mwhudson's suggestion to use systemd-run during gpg utility interaction to provide scoped references to any background services launched by gpg. Then cloud-init will clean up the known cgroup when APT gpg key interaction is complete.

References:
[1] Traceback calling gpgconf:
16:53:35 AssertionError: ['(\'apt_configure\', ProcessExecutionError("Unexpected error while running command.\\nCommand: [\'16:53:35 FAILURE: __main__.CloudInitTests.test_cloud_init_has_no_captured_failures
16:53:35 Traceback (most recent call last):
16:53:35 File "/home/ubuntu/tests.py", line 45, in test_cloud_init_has_no_captured_failures
16:53:35 self.assertFalse(
16:53:35 AssertionError: ['(\'apt_configure\', ProcessExecutionError("Unexpected error while running command.\\nCommand: [\'gpgconf\', \'--kill\', \'all\']\\nExit code: -\\nReason: [Errno 2] No such file or directory: b\'gpgconf\'\\nStdout: -\\nStderr: -"))'] is not false : cloud-init has reported errors. Please investigate. ['(\'apt_configure\', ProcessExecutionError("Unexpected error while running command.\\nCommand: [\'gpgconf\', \'--kill\', \'all\']\\nExit code: -\\nReason: [Errno 2] No such file or directory: b\'gpgconf\'\\nStdout: -\\nStderr: -"))']
16:53:35\', \'--kill\', \'all\']\\nExit code: -\\nReason: [Errno 2] No such file or directory: b\'gpgconf\'\\nStdout: -\\nStderr: -"))'] is not false : cloud-init has reported errors. Please investigate. ['(\'apt_configure\', ProcessExecutionError("Unexpected error while running command.\\nCommand: [\'gpgconf\', \'--kill\', \'all\']\\nExit code: -\\nReason: [Errno 2] No such file or directory: b\'gpgconf\'\\nStdout: -\\nStderr: -"))']
16:53:35