fix: Allow icon metadata to be file

[e-minguez]

Fix #34

[e-minguez]

For more context, there are helm charts like https://github.com/rancher/charts/blob/dev-v2.12/charts/longhorn/106.3.0%2Bup1.9.1/Chart.yaml#L17 where the icon is a file:

...
apiVersion: v1
appVersion: v1.9.1
description: Longhorn is a distributed block storage system for Kubernetes.
home: https://github.com/longhorn/longhorn
icon: file://assets/logos/longhorn.png
keywords:
...