Build separate images for apmsoak and apmbench

As of #24, we have 2 commands with `apm-perf`: `apmsoak` and `apmbench`. Consider using buildkite with [ko image builder](https://github.com/ko-build/ko) ([ref](https://github.com/elastic/apm-perf/pull/24#pullrequestreview-1597578532))

Start by reviewing issue #24 and its referenced pull-request discussion to understand how apmsoak and apmbench currently share the apm-perf image. Then examine the proposed Buildkite and ko image-builder approach; done means the two commands are built and published as separate images.

Written by the indexing model from the issue text.