feat: stacks and demo supports OCI helmCharts

[NickLarsenNZ]

Fixes #417

Note

This was bugging me enough when trying to validate the observability stack (which before this requires a bunch of manual steps).

You can now refer to Helm Charts stored in OCI.

Eg: in a stack/demo yaml:

- helmChart: release-file.yaml

release-file.yaml:

releaseName: nifi-operator
name: nifi-operator
repo:
  name: nifi-operator
  url: oci://oci.stackable.tech/sdp-charts # 👈 yeah boi
version: 0.0.0-dev
options: {}

Here's an example:

I also tested to make sure existing stacks still work (they do):

cargo stackablectl stack install logging

[Techassi]

LGTM, left a few small comments.