Support externally defined providers

[vetsin]

So that one may write custom providers of which are 'internal', a good example would be how docker does it:

• https://github.com/docker/docker-credential-helpers
• You may configure a provider, e.g. { "credsStore": "ecr-login" }
  • This would use the ecr-login provider, as written here: https://github.com/awslabs/amazon-ecr-credential-helper

So in teller world that may be:

providers:
  ecr-1: 
    kind: ecr-login

or more likely:

providers:
  ecr-1:
    kind: credential-helper
    bin: ecr-login

The idea being we use use stdin/out from other binaries as the interface, optionally.

Something like ProviderKind::CredentialHelper which literally just calls format!("teller-credential-{} {}", bin, action)? Albeit without the command injection

This may also allow fancier credential syncing where we may have needed a template before, e.g. if i wanted to setup a docker registry auth where our secret is an arbitrary provider, we may teller put it by essentially calling docker login ${registry:-} -u ${user} --password-stdin. I think before i would have needed docker-temp.t and then overloaded DOCKER_CONFIG locations, or some shell logic.