downloader: support for downloading bundles from an OCI registry (#4558)

Initial support for #4518.

Configuration uses the 'services' config for registries, via the "type: oci" field.
Bundles configured to pull from that service will then use OCI.

```
services:
  ghcr-registry:
    url: https://ghcr.io
    type: oci
bundles:
  authz:
    service: ghcr-registry
    resource: ghcr.io/${ORGANIZATION}/${REPOSITORY}:${TAG}
    persist: true
    polling:
      min_delay_seconds: 60
      max_delay_seconds: 120
persistence_directory: ${PERSISTENCE_PATH}
```

Service credentials are supported: if you want to pull from a private registry,
use
```
services:
  ghcr-registry:
    url: https://ghcr.io
    type: oci
    credentials:
      bearer:
        token: ${GH_PAT}
```

If no `persistence_directory` is configured, the data is stored in a directory under /tmp.

See docs/devel/OCI.md for manual steps to test this feature with some
OCI registry (like ghcr.io).

Signed-off-by: carabasdaniel <dani@aserto.com>