`aws_guardduty_context` helper function does not include `accountId` in the alert context

[pbnj-dragon]

Problem

aws_guardduty_context helper function does not include accountId in the alert context

Proposed Solution

Improve the built-in aws_guardduty_context helper function to include the accountId

Workaround

Clone the managed detection to extend the aws_guardduty_context in the alert_context() function, like:

def alert_context(event):
    return aws_guardduty_context(event) | {"accountId": event.get("accountId", "<MISSING ACCOUNT ID>")}

Downsides:

1. This workaround does not scale well (panther users have to clone many panther-managed detections)
2. Lose out on future updates to panther-managed detections