feat: expand hub support to helm plugins

[andyfeller]

The same motivation behind the creation of Helm Hub of enabling distributed search in an intentional way applies Helm plugins, which is a mechanism for adding functionality to Helm independently of Helm release cycles. While there is minimal documentation on the Helm website how to begin searching for plugins, this has the same pains and benefits that led to making charts discoverable.

[scottrigby]

Closing this since we have an artifact hub issue where we can follow up on this.

[tegioz]

Artifact Hub now supports Helm plugins :)

https://artifacthub.io/docs/topics/repositories/#helm-plugins-repositories