Make this native to Helm?

[jdolitsky]

Just tried this out for generating a provenance file, and appeared to work better than Helm's built-in provenance (at least w/ my key setup).

Does it make sense to add sign and verify subcommands directly in Helm?

[jdolitsky]

For context, tried adding a .prov file to chartmuseum chart repo, but it doesn't actually work with --verify.

See caveat here: https://github.com/chartmuseum/charts/releases/tag/chartmuseum-2.16.0