Support authentication

[jchesterpivotal]

Description

Currently, the resource only works for public jobs in public pipelines.

This is rather limiting.

Something like:

- name: build
  type: concourse-build
  source:
    concourse_url: https://concourse.example.com
    authentication:
      # ???

Open questions

• How does this actually work? Authentication has lots of knobs and dials. Presumably I would need to implement some logic per scheme -- one for github, one for CF auth etc.
• Limit to 4.x and above? Authentication changed radically in 4.0, which is why there's a 4.0.
• How would tokens work? Do we log in every time?
• What happens when authentication fails?
• What information is recorded about auth? Do I add it to version metadata? Print it to stderr? Create more K-V files? All of these?

Todos

Assuming I focus on github only and split out other auth options into standalone issues.

• check
  • cmd/check
  • pkg/check
• in
  • cmd/in
  • pkg/in
• Readme

[jchesterpivotal]

A note on supported versions. Given my intention in #28 to rely entirely on upstream go module definitions, it won't be possible to support this feature for Concourse below (approximately) 4.2.1. Given that it's just me working on this, I am totally comfortable with that policy.

[jchesterpivotal]

Marking blocked on #28.