AWS Lambda: Add option to opt-out of special handling of the "payload" field

[mrjv]

Note: This ticket is a duplicate of the already closed #1154, but was raised again for visibility (I don't have permissions to reopen closed tickets). - Feel free to close it and reopen the original ticket instead.

The AWSLambdaUtils.java currently have special handling for Lambda payloads containing a "payload" field:

spring-cloud-function/spring-cloud-function-adapters/spring-cloud-function-adapter-aws/src/main/java/org/springframework/cloud/function/adapter/aws/AWSLambdaUtils.java

Lines 127 to 130 in 1fc0fe3

	MessageBuilder builder = MessageBuilder
	.withPayload(structMessage instanceof Map msg && msg.containsKey("payload")
	? (msg.get("payload"))
	: payload);

The following Lambda payload:

{
  "someProperty": "someValue",
  "payload": {
    "otherProperty": "otherValue"
  }
} 

results in only {"otherProperty": "otherValue"} being passed to the Spring cloud function. We would like to opt out from this behavior and are on a high level looking for something like the following:

 MessageBuilder builder = MessageBuilder 
 		.withPayload(SOME_PAYLOAD_EXTRACTION_FLAG && structMessage instanceof Map msg && msg.containsKey("payload") 
 				? (msg.get("payload")) 
 				: payload); 

Where we would set SOME_PAYLOAD_EXTRACTION_FLAG to false using a configuration or similar. Completely removing the special logic would also be fine, but it was probably added for some reason?