PipelineContext replaced with PipelineResult

[KathleenDollard]

Previously, PipelineContext was ephemeral, subsystems carried input specific caches (oops), was often recreated, and the return was CliExit.

Now, PipelineResult carries all three purposes and a single instance is used for the life of the pipeline run (per call to Execute)

This is currently in draft state because it will have merge conflicts with #2413, includes all the commits from #2413, and because it needs a cleanup/my review pass Edit: Now up to date with those PRs

[mhutch]

Changes look ok. I do wonder whether we might want the pipeline to return a result type that wraps the pipeline state object, is immutable and doesn't have all the members available to the subsystems. In that case the name PipelineResult might be better reserved for that wrapper type. But that would be easy to change anytime before we freeze the API, so let's merge the PR as-is.