Support file-watching on a per-artifact basis

Some language runtimes, like the JVM, support dynamic code replacement during debugging (known by different terms such as _live-edit_, _hot-swap_, _hot-reload_). For such language runtimes, `skaffold dev`'s rebuild-and-redeploy (aka _file watching_) on a source change causes the corresponding container to be torn down and any processes being debugged to be restarted. This is disorienting and annoying for developers used to dynamic code replacement, and so `debug` disables this file-watching behaviour.

For languages runtimes that do not support dynamic code replacement, or where ecosystems have become sufficiently complicated that code replacement is not safe (notably the JavaScript ecosystem), applying source changes does require a rebuild and redeploy.

To support mixed-language microservice scenarios, Skaffold should support specifying and manipulating the file-watch status on a per-artifact basis.