Support specifying credentials via a path

[preslavgerchev]

Is your feature request related to a problem? Please describe.
A follow-up to #78, we should provide a way to specify the Mondoo creds via a file (path). currently, the mondoo creds are only supported by specifying them directly as a JSON input.

We should update the provider to support both:

provider "mondoo" {
  region = "us"
  credentials = "{....}" // if set, we read this
  credentials_path = "/tmp/mondoo-creds" // if set, we read this
}

We need to only define behaviour when both are specified. Do we throw an error or do we have a defined order of precedence?