feat: added method for generating uniq hash

[pandwoter]

Hello, I'm planning to use this method for implementing hash generation in particular providers as follows

def kubeconfig_path
  File.join(
    kubeconfig_dir,
    "#{environment.app_name.downcase}" \
    "-#{generate_hash(config.access_token + config.cluster_id)}" \
    "-kubeconfig.yaml"
  )
end

Also, I created PR for digital ocean provider (getkuby/kuby-digitalocean#3)
So if the implementation is ok, let me know and I will do it for other providers too

(this PR is a part of #40)

[camertron]

Please see comments on getkuby/kuby-digitalocean#3. TL;DR is I'd like to see the generate_hash function defined in the provider gems instead of in the Provider superclass.

[pandwoter]

Implemented in Provider superclass