JMESPath Support

Not every JWT token provider is able to define the token the way the chartmuseum wants it.
In my case, my sso produces e.g.:

"resource_access": {
    "chartmuseum": {
      "roles": [
        "pull"
      ]
    }
  },

If you are not able to configure the JWT token provider yourself, i think it is a good idea to provide a solution by the chartmuseum auth module.

With JMESPath Support we can make the path to find the allowed actions configurable.
To avoid a breaking change, i defined a default JMESPath. This default configuration does the same job as the old implementation.

The pull request for the chartmuseum repository: helm/chartmuseum#381