Support username and password tokens from dockercfg auths

[tpokki]

Environment:

• Jib version: 3.1.2
• Build tool: maven 3.6.3
• OS: macOS

Description of the issue:
Jib is missing support for username and password fields in dockercfg auths block.
Example

{
  "auths": {
      "registry.example.com": {
           "username": "someuser",
           "password": "somepw"
       }
}

Expected behavior:
Jib to pick up username and password fields from dockercfg (and not just credHelper and auth).

Steps to reproduce:

1. Add username and password for your registry into your config.json as described above
2. Build
3. Jib does not pick up username and password

Additional Information:

I have implemented the necessary changes to add support in here:
tpokki@479a5df

If you approve, I'll create PR.

Thanks,
Toni

[chanseokoh]

I think this is great. Create a PR (with a test), and we'll review it. Thanks a lot!

[chanseokoh]

Fixed by #3365.

[chanseokoh]

@tpokki we released Jib plugins 3.1.4 with the fix.

[chanseokoh]

Thanks for your contribution!