feat(stackable-operator): Support gitsync via SSH

[adwk67]

Description

Prerequisite for stackabletech/airflow-operator#382.
Tested with stackabletech/airflow-operator#718.
The gitsync test in Nifi was also successfully tested with this PR.

CRD Change

See https://github.com/stackabletech/decisions/issues/68

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.
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible

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

[sbernauer]

Only really looked at the CRD change (as the decision was accepted) and left a suggestion.
IIRC we wanted to try CRD versioning for this, sadly that will be another beast to tackle
Pls fee free to ping @Techassi, @NickLarsenNZ (or me if they are absent) if you need any help with that!

Because we should have CRD versioning in place before merging this (as otherwise it breaks v1alpha1)

edit: Oh sorry, I thought this was Waiting for review, my fault!