new stage: emrserverless

[cnfait]

Issue #, if available:
#309

Description of changes:
Definitely not stable, but a good start anyway: run an EMR Serverless job in a stage. Like #338 and #342 it supports three different triggers:

• event: run stage when an event received on the team's event bus matches the configured event pattern
• event-schedule: store events received on the team's event bus matching the configured event pattern, then process them on the configured schedule
• schedule: run stage on the configured schedule, without any event as input

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