add-on `post_install_actions` should be separately runnable (`ddev add-on --reapply` ?)

[rfay]

Is your feature request related to a problem?

We tell people to ddev add-on get an extra time for any logic to be applied.

For example, in ddev-platformsh, if your Platform.sh config has changed, you have to ddev add-on get ddev/ddev-platformsh again, even if there has been no change upstream.

This is a result of bunding the post-install actions into the add-on, and not having another way to run them

Describe your solution

Unbundle the post-install into a separate command, perhaps ddev add-on --reapply <add-on>

Add the logic of that command in the post_install_steps

Describe alternatives

No response

Additional context

No response