create credential conveniences

[jakecoffman]

When testing with credentials it's a bit redundant to have to specify the metadata in the job definition:

job:
  package-manager: docker
  allowed-updates:
    - update-type: all
  source:
    provider: github
    repo: dependabot/test
    directory: /docker
  credentials-metadata:
    - type: docker_registry
      registry: example.com
      replaces-base: true
    - type: docker_registry
      registry: example.com
credentials:
  - type: docker_registry
    registry: example.com
    username: user1
    password: $PW1
  - type: docker_registry
    registry: example.com
    username: user2
    password: $PW2

With this change, the following is equivalent to the above:

job:
  package-manager: docker
  allowed-updates:
    - update-type: all
  source:
    provider: github
    repo: dependabot/test
    directory: /docker
credentials:
  - type: docker_registry
    registry: example.com
    username: user1
    password: $PW1
    replaces-base: true
  - type: docker_registry
    registry: example.com
    username: user2
    password: $PW2

The CLI will automatically add the credentials-metadata section. This is more inline with how the dependabot.yml works.

I've also made the Credentials definitions all match up with a proper type rather than specifying a map everywhere.

[mattt]

I think this is a nice convenience. Nice work, @jakecoffman!

The one concern I have is that automatic behavior becomes invisible and become unexpected. What do you think about logging a debug / warning message giving a quick heads-up when the CLI makes these changes to a job definition?

[jakecoffman]

@mattt makes sense. Along those lines I also added the injected credentials into the output so when specifying -o you end up with what was actually run.

[jakecoffman]

I'm going to run with this locally for a bit longer to make sure there aren't any issues.