feat(pipeline/configuration): Updated pipeline level configuration that introduces a new property named metadata.

[tonyg4864]

SpEL processing pipeline.metadata so that it is resolved/ready when referenced.

[dbyron-sf]

What's the motivation for this change?

[tonyg4864]

What's the motivation for this change?

1. The original motivation is a CDEvents Pipeline level notification. I needed to send a Pipeline Level CDEvent notification with a payload that includes data from stages. Example : A Pipeline Level CDEvent Notification that includes data from all failed stages. Do not want to need to add a notification config to each specific stage to be able to accomplish this. Also I want the notification to be sent once at the end of the pipeline. Current functionality forces event notification at stage level leading to multiple events being sent every time a stage fails. The notification should be able to access Pipeline level constants/properties(metadata) avoiding the need to have successfully executed a "Evaluate Variables" stage.
2. Second motivation is to be able to include pipeline wide properties/constants that can be referenced anywhere without having to run a "Evaluate Variables" stage.