[feature] bk run - allow running local pipeline using remote agents

[drnic]

Whilst discovering https://github.com/buildkite/agent/issues/1181 I came across the desire for bk run or bk local run to split out its behaviour and run a local pipeline, but use remote agents.

I like bk local run -- it allows me to test a new pipeline before I "commit" all future PRs/commits/team members to the pipeline changes. You can iterate on the pipeline.

The command happens to run the steps locally; though to me this is tangential.

I can imagine this is not a small PR. I wanted to raise the topic and perhaps someone can contribute if its a good or bad idea.