Refactor `--http-probe-exec` and `--http-probe-exec-file` to be separate from HTTP probes and add its own `continue-after` mode

[kcq]

These flags represent app probes that you configure to run when docker-slim is doing dynamic analysis. It should be done separately from the existing HTTP probes.

New names: --app-exec (was --http-probe-exec) --app-exec-file (was --http-probe-exec-file)

New continue-after mode: app

[rikez]

Happy to work on it. I think it's a good one to become more familiar with the codebase. and the features.

[kcq]

@rikez great! also happy to do a code walk through if you'd like :)

[rikez]

Quick update: still working on that, will put a PR soon.

[kcq]

Quick update: still working on that, will put a PR soon.

Any questions or thoughts? Let me know if you need any help.