Access the values of attributes from get method

Use Case

Allow credentials to be passed to the provider without storing those on the local machine

Describe the Solution You Would Like

    vault_secret_engine { 'nomad':
      type        => 'nomad',
      ensure      => 'present',
      description => 'dynamic token management for nomad',
      vault_token => 'my super token',
    }

Writing providers like this one for vault or even the consul one (https://github.com/solarkennedy/puppet-consul/blob/master/lib/puppet/provider/consul_policy/default.rb) some times require different tokens for each type of operation, the issue is that you don't have access to attributes during get or prefetch and it leads to hacks to make it work. I'd like have access to specific values so it become possible to use these values on API requests without the need to a transport.

Start with the provider examples in the issue, especially vault and consul, then trace how get and prefetch currently receive resource data in this repository. Define the expected attribute access for provider API requests and verify the behavior with focused coverage; done means providers can use per-resource credentials without storing them locally or requiring a transport.