add support for oci push

[monotek]

Add OCI registry push support via cr push

Adds three new inputs that let the action push packaged charts to an OCI registry, alongside (or instead of) the existing GitHub Pages flow:

• oci_registry_url — when set, triggers an OCI push (e.g. oci://ghcr.io/myorg/charts)
• oci_username — optional, falls back to the local registry credential store
• oci_password — optional, falls back to the local registry credential store

Under the hood, this wires a new push_charts() function in cr.sh that invokes cr push after cr upload (in both the packaging and skip_packaging code paths). The existing skip_existing input is reused for cr push --skip-existing.

README adds the inputs to the reference table and ships a worked example showing the recommended auth flow via docker/login-action before the action runs (so credentials land in ~/.docker/config.json and cr push picks them up automatically).

⚠️ Mandatory dependency

This PR cannot be merged until helm/chart-releaser#622 is merged and released as a tagged version of chart-releaser. The new cr push command does not exist in any released version yet — using this action with the default version: v1.7.0 will fail with unknown command "push".

After #622 ships, the default version in action.yml should be bumped to the release that contains cr push as a follow-up.