Support explicit extract step

[alex-statsig]

Not sure if this is within the realm of what this action intends to support, but I think theres a valid use-case for wanting an explicit step which extracts a folder from the build. Currently, it is only possible to have a "post" extraction step as part of the combined inject/extract action.

My use case is to extract files for upload to a CDN. In this case, I want to run an explicit upload action after the extraction step. However, since the extract currently runs in the "post" stage after all "non-post" actions, this is hard to do cleanly (its very much designed to be used with a caching action that also runs during the "post" stage).

I saw this was possible in v1, but removed in v2 in favor of the simpler combined model.