Add a flag to keep the created simulator around with `bazel run`

Right now in some configurations the [script](https://github.com/bazelbuild/rules_apple/blob/10b0699496e438435d74b68f27b2b86355d913a9/apple/internal/templates/ios_sim.template.py) creates a simulator for running and deletes it afterwards. This is annoying if you're attempting to iterate on a change.

I imagine most folks today are using some sort of Xcode integration so this is a minor issue, but I think it would be fine to have a flag to specify to keep this (or maybe the default is even to keep it and there's a flag to make them ephemeral I'm not sure)

Related to https://github.com/bazelbuild/rules_apple/issues/225

Start with apple/internal/templates/ios_sim.template.py and trace how the simulator is created and removed when invoked through bazel run. Review related issue #225 for context on the intended behavior. Done means the issue has a documented choice of default or flag behavior that lets users keep the created simulator when requested.

Written by the indexing model from the issue text.