Switch to kubernetes/git-sync for datadir initialization

[edevosc2c]

Currently, we use a raw git command for cloning the datadir: https://github.com/georchestra/helm-georchestra/blob/main/templates/_bootstrap-georchestra-datadir.tpl

I recently came across an official way of git cloning inside Kubernetes: https://github.com/kubernetes/git-sync

This script has the benefit of offering many useful features for us:

• easy known hosts file verification: --ssh-known-hosts-file. related to https://github.com/georchestra/helm-georchestra/issues/36
• automatic retry in case the git server is not available, or an error occurred at the time of the clone: --max-failures
• ability to set user permissions for the clone: --add-user. may be useful for setting the same UID as the main container
• ability to set credentials using a json config: --credential. may be useful for supporting HTTPS (not SSH) based repositories
• ability to use HTTP proxies. may be useful for environments with strict security requirements. https://github.com/kubernetes/git-sync/blob/master/docs/proxy.md
• all the options are available through environment variables, so anyone can configure more options without clogging up the helm chart with all the different git options.