Expose a timestamp as a PAC variable

[arewm]

When processing the result of a pipelinerun, users may want to know the timestamps of the commit/pipelinerun in order to compare the freshness of produced artifacts. It would be nice to have PAC be able to inject a timestamp into the PLR so that it can be used for comparison. Two options are:

• The timestamp of the commit. This can be overridden by users when committing so it might not be strictly monotonically increasing with successive commits in a branch
• The timestamp of the trigger event. This would be beneficial if the system is under load so tasks might take longer to complete, increasing the discrepancy variability if the timestamp is determined within a task in the triggered pipeline