fix(pipeline): allow mid-task spel evaluation to reference prior stage outputs

[maggieneterval]

Closes spinnaker/spinnaker#4932

• chore(pipeline): add buildExecutionContext test to ContextParameterProcessorSpec

  Adds test to demonstrate that currently, the context returned by ContextParameterProcessor.buildExecutionContext, when used as the context for ContextParameterProcessor.process, does not allow for the evaluation of variables that refer to the outputs of a prior stage.

• fix(pipeline): allow mid-task SpEL evaluation to reference prior stage outputs

  Changes ContextParameterProcessor.buildExecutionContext to return StageContext, which extends a ForwardingMap in order to override get to search the stage's ancestors' outputs if a given key does not exist on the delegate object. Uses the Map previously returned by this method as the delegate. Removes the includeStageContext argument since all existing consumers want stage context included. Updates ExpressionAware interface to use updated buildExecutionContext per todo.

To do: Investigate whether tasks like ExpressionPreconditionTask even need to explicitly take another pass at expression evaluation, since presumably any resolvable variables were evaluated in the StartTaskHandler. I think only if the task itself fetches an artifact that may contain SpEL should we need another evaluation pass?

[marchello2000]

❤️ You are brave soul, @maggieneterval, for tackling this :)

[marchello2000]

lgtm!

[ezimanyi]

🔣🎭📤🎉 💯