add support for on_exit deferring it for all the commands

[umputun]

The goal is to add a per-task on_exit command that will be invoked on the task's completion on each server. The goal is to allow all kinds of cleanup actions, service restarts, and so on to be in a more logical place.

Currently, it is possible to add some cleanup commands, but for larger tasks, it is hard to follow, and having it right in the command should make it easier.

For example:

task:
  - name: "copy script"
    copy: {src: "testdata/script.sh", "dst": "/tmp/script.sh", "chmod+x": true}
    on_exit: "rm -fv /tmp/script.sh" # register deferred action to remove script.sh after execution

  - name: "run script"
    script: "/tmp/script.sh"

Here, we "register" the removal of /tmp/script.sh. This is somewhat similar to Go's deffer running the command on function completion, but in the case of Spot, it runs on task completion instead and on each remote server.

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