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

Previously the optional credentialsSecret was used for username & password/token:

...
  clusterConfig:
    dagsGitSync:
      - repo: https://github.com/stackable-airflow/dags
        credentialsSecret: git-credentials
...

It is now possible to also connect via ssh using sshSecret:

  clusterConfig:
    dagsGitSync:
      - repo: ssh://[email protected]/stackable-airflow/dags.git
        sshSecret: git-sync-ssh

i.e. the optional field sshSecret has been added. Either credentialsSecret or sshSecret can be specified, but not both (an exception is thrown if this is detected).

Alternatives considered

• introduce a complex enum e.g. accessSecret, holding either credentialsSecret or sshSecret, holding either types of secret
• keep credentialsSecret as-is but use it to hold the kind of secret determined by the scheme (https vs. ssh)

In both cases the change is breaking and/or the naming is unhelpful and the usage is slightly different from what is documented by gitsync itself.

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

[Techassi]

note: Ideally we want this to be a single field instead which is an enum with two variants. This completely prevents the potential misuse by users.

I know this is a breaking change, but we should consider it going forward.