Add new keyword to support running after_script for canceled jobs

See Epic &10158 (closed) for full problem and solution description.

Step	Issue
Backend, Step 1	#398774 (closed)	<- You are here
Runner, Step 1	gitlab-runner#30827 (closed)
Backend, Step 2	#399215 (closed)
Frontend	#399294 (closed)
Backend, Step 3	#399296 (closed)
Feature Flag Rollout	#399302 (moved)

Proposal

Backend, Step 1

NOTE: Gitlab communicates with the runner through the Runner API - resource :jobs endpoint.

A new keyword should be added to minimize the impacts to existing users so that they can opt-in the fix. See the following example:

job:
  after_script:
    script:
      - echo "Execute this command after the `script` section completes."
    when: always

Runner will receive the new keyword when: always via the API

Implement this behind a feature flag which will be used for all related issues for the epic.

• Update related issues with the name of the feature flag to be used.

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