Feat/multi git sync

[Maleware]

Description

Part of #721

This PR adds support for multiple instances of git-sync.

This is done via a symlink onto the current folder of each git-sync instances.

In kubernetesExecutors a cp -r replaces the symlink as it is a one off anyways and it's easier to transition contents via volumeMounts and init-container.

Integration test 'mount-dags-gitsync' has been changed to use two git-sync resources at different branches of the same repository.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[Maleware]

All tests passed locally apart from

--- FAIL: kuttl/harness/logging_airflow-3.1.6_openshift-false_executor-kubernetes (801.68s)
--- FAIL: kuttl/harness/logging_airflow-3.1.6_openshift-false_executor-celery (1022.60s)

which is a known issue and will be tackled separately.

[Maleware]

RELEASE-NOTE-SNIPPET:

Airflow supports now multiple git-sync instances and thus multiple git repositories ( or multiple branches from one repository ). Note that the default path has changed while using git-sync to /stackable/app/allDAGs/current-{i}