devel/buildkite-agent: NEW PORT

The Buildkite agent is a small, reliable and cross-platform build runner
that makes it easy to run automated builds on your own infrastructure,
using the proprietary BuildKite application and infrastructure.

Its main responsibilities are polling buildkite.com for work, running
build jobs, reporting back the status code and output log of the job,
and uploading the job's artifacts.

Sponsored by: SkunkWerks, GmbH
Differential Revision: https://reviews.freebsd.org/D35043