feat: add support for SNS->SQS protocol

[heitorlessa]

Issue #, if available: #265

Description of changes:

When using SNS -> SQS -> Lambda, the SNS payload differs from SNS -> Lambda. It's not explicitly documented as per #262 - This information is available on per Target protocol, and Lambda is the only one that seems to differ from others (e.g. SQS, HTTP, etc.)

This PR does the following:

• Adjusts MessageAttributes to be an optional field
• Dynamically adjusts UnsubscribeURL -> UnsubscribeUrl, and SigningCertURL -> SigningCertUrl to make backwards compatible
• Creates a new envelope SnsSqsEnvelope to make this process seamless for customers

UX

Excerpt from tests, where customers can continue to write their parsing as before, but use a new envelopes.SnsSqsEnvelope to correctly extract the original SNS payload that was doubled marshalled too.

@event_parser(model=MySnsBusiness, envelope=envelopes.SnsSqsEnvelope)
def handle_sns_sqs_json_body(event: List[MySnsBusiness], _: LambdaContext):
    assert len(event) == 1
    assert event[0].message == "hello world"
    assert event[0].username == "lessa"

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

RFC issue #:

• Migration process documented
• Implement warnings (if it can live side by side)

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[nmoutschen]

LGTM - just 2 small remarks

[heitorlessa]

responded to comments; merging

[heitorlessa]

responded to comments; merging