Allow signing of packages

[mrsimonemms]

Is your feature request related to a problem? Please describe.
I've been looking at using this library to publish signed OCI charts. The Helm docs require passing additional arguments to the helm package command to achieve this

helm package --sign --key 'John Smith' --keyring path/to/keyring.secret mychart

Describe the solution you'd like
There are two ways of achieving this to my mind:

1. add some arguments to the input - I think it'll be something like sign, key, passphrase and keyring with sign defaulting to false (passphrase would emulate passphrase-file - to read from stdin)
2. add ability to add any argument to the package command. Whilst I'm not normally a fan of this approach, it might be useful to allow any argument to be passed in for future usage?

In both cases, the setting up of the key/keyring will be external to this. The PR should also include updating the README to include a signing example

Describe alternatives you've considered
Create my own Helm publish Action

Additional context
Happy to create a PR if this would be acceptable.