Overload manager: per-timer triggers

Title: * Overload manager: per-timer triggers*

Description:

It would be really useful if in the reduce_timeouts action, you could configure different triggers for each timer.

Currently, it's only possible configure a set of triggers for the whole reduce_timeouts action which then uses the max pressure across all the triggers.

Imagine if you could configure something like this:

actions {
  name: "envoy.overload_actions.reduce_timeouts"
  timer_scale_factors {
    timer: HTTP_DOWNSTREAM_CONNECTION_IDLE
    min_timeout {
      seconds: 1
    }
    triggers {
      name: "my.custom.resource1"
      ...
    }
  }
  timer_scale_factors {
    timer: HTTP_DOWNSTREAM_CONNECTION_MAX
    min_timeout {
      seconds: 1
    }
    triggers {
      name: "my.custom.resource2"
      ...
    }
  }
}

This would give you more granular control of each timer.

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