Support multiple functions per AWS event ARN, merge SNS filter policies

[ellen-goc]

Overview

Two related improvements to Zappa's event-source handling that address limitations when multiple Lambda functions share trigger sources:

1. Multiple functions per AWS event ARN

Currently AWS_EVENT_MAPPING maps a single ARN to a single function. This means two functions listening to the same SQS queue, SNS topic, DynamoDB stream, or EventBridge rule require separate ARN entries and cannot share a trigger without duplicating infrastructure.

Proposed change: AWS_EVENT_MAPPING maps an ARN to a list of functions, with backward-compatible handling for the existing single-string format.

2. Merge (not overwrite) SNS filter policies

Currently SNSEventSource.update() overwrites the subscription's filter policy on each zappa update run. This discards any manually-added filter conditions from previous runs.

Proposed change: SNSEventSource.update() merges the new filter policy with the subscription's existing filter policy instead of replacing it entirely.

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Related PR

A draft implementation exists at #1462 (currently conflicting with master; needs rebase before review).

Acceptance Criteria

• AWS_EVENT_MAPPING accepts both a single function name (string, existing format) and a list of function names for a given ARN
• All functions listed for an ARN are registered as event consumers during zappa deploy/zappa update
• SNSEventSource.update() retrieves existing filter policy attributes and merges the new policy on top rather than replacing it
• pytest tests/test_handler.py tests/test_utilities.py passes