Add support for microk8s minio addon

[carlcsaposs-canonical]

Needed for s3 storage to test some spark functionality (needed to use reusable integration test workflow on https://github.com/welpaolo/spark-configuration-hub)

microceph is insufficient since it is "really heavy to run locally"

Originally requested by @welpaolo

[github-actions]

https://warthogs.atlassian.net/browse/DPE-4086

[carlcsaposs-canonical]

used in almost all (but not every) integration test on https://github.com/welpaolo/spark-configuration-hub

note to self: probably should implement with the same UX as microceph pytest plugin

(would replace current method of retrieving credentials in a bash script
https://github.com/welpaolo/spark-configuration-hub/blob/00ccb7ef1e4e6a5d5cc110e367708e6e3056f51a/tests/integration/test_charm.py#L140
https://github.com/welpaolo/spark-configuration-hub/blob/00ccb7ef1e4e6a5d5cc110e367708e6e3056f51a/tests/integration/setup/setup_minio.sh
)