Add BuildKite support to Athenz

[jonmv]

It would be great to have access to Athenz credentials through siad in BuildKite, as in Github Actions.

#2706 adds the ZTS InstanceBuildKiteProvider and the sia-buildkite SIA daemon required for this. it's possible a plugin could be added to BuildKite as well, but we'll probably set up and run siad in a different way ourselves.