Add configurable runs to execute external command before the skaffold runner creation

- Skaffold should provide a way for users to run external command before whole configuration parsing/ skaffold run context creation
- It would be very helpful if skaffold can parse the output of the external commands, then make the output available as environment available in the skaffold run session.
- skaffold should also provide a way to namespace an external command run,
- For example, "name":"abc", "run":"xxxx", "parseOutput":"true"
- Then users should be able to references the any fields from output in a skaffold templated fields, like '{{.abc.somefiled}}'
- This types of setup should be activate through command line, like skaffold command --run-before=abc,def..
- This before run is also useful when users need to set up somethings, for their builds. Examples, https://github.com/GoogleContainerTools/skaffold/issues/7469 , https://github.com/GoogleContainerTools/skaffold/issues/7183, https://github.com/GoogleContainerTools/skaffold/issues/7216

- skaffold may also need to have a way to execute external commands after session run for doing cleanup things.