### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

localstack v0.12.17 released with this enhancement that downcases all headers:

> canonicalize HTTP header names in API GW Lambda events to lower-case
> https://github.com/localstack/localstack/commit/657d9fe4bd0e3c4284e6fa67aec62d265d5d9fff

This does not match what we observe in production - custom headers are not downcased. See this part of the docs on API Gateway and how it canonicalizes headers:

> API Gateway enacts the following restrictions and limitations when handling methods with either Lambda integration or HTTP integration.
> * Header names and query parameters are processed in a case-sensitive way.
> * The following table lists the headers that may be dropped, remapped, or otherwise modified when sent to your integration endpoint or sent back by your integration endpoint. In this table: 
> https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-known-issues.html

### Expected Behavior

I would expect that localstack follows the same canonicalization rules that API Gateway implements.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack:0.12.17

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

Used [Insomnia](https://insomnia.rest/) client to issue a POST request to an AWS Lambda function via API Gateway with custom headers. For example, `X-A-Custom-Header`. Attempt to read the custom header in your lambda function case sensitively.


### Environment

```markdown
- OS: macOS for POST request + whatever the docker container uses for localstack
- LocalStack: 0.12.17
```


### Anything else?

_No response_
queue_arn=queue_name,
            path=self.API_PATH_DATA_INBOUND,
        )
        
        self.create_lambda_function(self.TEST_LAMBDA_SQS_HANDLER_NAME)
        event_source_data = {
            "FunctionName": self.TEST_LAMBDA_SQS_HANDLER_NAME,
            "EventSourceArn": aws_stack.sqs_queue_arn(queue_name),
            "Enabled": True,
        }
        add_event_source(event_source_data)
        
        test_data = {"spam": "eggs & beans"}
        url = gateway_request_url(
            api_id=result["id"],
            stage_name=self.TEST_STAGE_NAME,
            path=self.API_PATH_DATA_INBOUND,
        )
        result = requests.post(url, data=json.dumps(test_data))
        self.assertEqual(200, result.status_code)
        parsed_json = xmltodict.parse(result.content)
        result = parsed_json["SendMessageResponse"]["SendMessageResult"]
        body_md5 = result["MD5OfMessageBody"]
        self.assertEqual("b639f52308afd65866c86f274c59033f", body_md5)
        
        sqs_client = aws_stack.create_external_boto_client("sqs")
        sqs_client.delete_queue(QueueUrl=queue_url)
        lambda_client = aws_stack.create_external_boto_client("lambda")
        lambda_client.delete_function(FunctionName=self.TEST_LAMBDA_SQS_HANDLER_NAME)
    def test_api_gateway_sqs_integration(self):