aquery action attrs should be strongly typed

When using aquery from bxl, and trying to get the command line args for a cxx_compile action, it’s a value like `[cl.exe, -c, foo.cpp, @argsfile, -o, foo.obj]`. Unfortunately, it’s quite literally a string, so to get it as a Starlark list you have to do the very awkward

```
cmd = cmd.strip(“[]”).split(“,”)
cmd = list(map(lambda x: x.strip(), cmd))
```

which is sure to break at the worst possible time. Is it possible to make the action_attr binding support more types than just raw strings?