feature(Execution): Allow ignoring failed stages

[AbdulRahmanAlHamali]

Goes with:
spinnaker/orca#4113
spinnaker/deck#9039

[spinnakerbot]

The following commits need their title changed:

• 0087aae: add ignore stage failure endpoint

Please format your commit title into the form:

<type>(<scope>): <subject>, e.g. fix(kubernetes): address NPE in status check

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