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