Add support for pushing Helm charts to OCI registries, rather than just Github Pages-based chart repos.

[bleggett]

As per #186

This adds support for publishing Helm charts to an OCI registry.

A configuration like

charts:
  - name: binderhub
    repo:
      oci: ghcr.io/jupyterhub
      prefix: helm-charts

will push the binderhub chart to to Github's GHCR OCI registry under the path ghcr.io/jupyterhub/helm-charts/binderhub

This should work for Amazon's OCI repo (ECR), Google's OCI repo (GCR), and Docker's self-hosted registry image, all of which are OCI-compliant registries.

Additionally, this PR adds support for a basePath key under chart definitions, for repos that keep charts under a specific directory structure - this is roughly analogous to contextPath for images.

This means chartpress.yaml can be kept in the repo root, and users have more freedom as to how and where charts and images are kept relative to each other, as well as to chartpress.yaml itself.

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