support ghcr to store envoy-gateway oci helm chart

[networkhermit]

Description:

Currently the envoy-gateway oci helm chart is published in dockerhub, this has some shortcomings:

1. dockerhub download is rate limited and could be easily blocked in environments which share a common public ip
2. sadly the dockerhub seems to be indefinitely blocked in China, making users and developers in CN more difficult to promote and adopt envoy-gateway

I think it's possible to publish oci helm chart in both dockerhub and ghcr, so users can benefit from either choice.

[optional Relevant Links:]

GHCR is extensively used by other projects such as prometheus community.