support using git-sync with PVC (allowing there to only be one git-sync container)

[thesuperzapper]

Currently, we don't let users enable dags.gitSync and dags.persistence at the same time.

If we do this (obviously requiring dags.persistence.accessMode = ReadWriteMany), we can massively reduce the number of git-sync containers required.

This is because we won't have to put a git-sync container in every Pod, but rather have all pods mount the PVC, while having a central single git-sync deployment that keeps the PVC up to date.

[justbeay]

in airflow1 version, gitSync with dags pvc works. we hope it keeps the same in airflow2. which would not leading the task container waiting for git clone done (this masters a lot if the git repo is too big or poor network connection)

[thesuperzapper]

@justbeay two comments:

1. version 8.X.X of the chart can be used with both airflow 1 and airflow 2 (see the Airflow Version Support table)
2. while the 7.X.X version of the chart may have let you enable a PVC with git-sync enabled, it would have resulted in all the git-sync containers fighting over the sync

[thesuperzapper]

I think this feature is really important for KubernetesExecutor, as right now if cloning the git repo takes a long time, then EVERY task will take at least that long, this is because the git-clone init-container must run before the task can, as it needs the DAG definitions.

[karakanb]

I have created a PR trying to address these, I'd appreciate feedback on it.