### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

We are using the `Authorization` header for routing purposes

```
 "authorization: AWS4-HMAC-SHA256 <...>"
```

https://github.com/localstack/localstack/issues/5770

### Expected Behavior

Use a custom header to avoid causing confusion with backend integrations

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

Run a custom HTTP integration without authorisation

### Environment

```markdown
- OS: 
- LocalStack:
```


### Anything else?

No
"accountId": account_id,
        "apiId": api_id,
        "resourcePath": resource_path or relative_path,
        "domainPrefix": invocation_context.domain_prefix,
        "domainName": invocation_context.domain_name,
        "resourceId": resource_id,
        "requestId": long_uid(),
        "identity": {
            "accountId": account_id,
            "sourceIp": source_ip,
            "userAgent": headers.get("User-Agent"),
        },
        "httpMethod": method,
        "protocol": "HTTP/1.1",
        "requestTime": pytz.utc.localize(datetime.datetime.utcnow()).strftime(
            REQUEST_TIME_DATE_FORMAT
        ),
        "requestTimeEpoch": int(time.time() * 1000),
        "authorizer": {},
    }
    
    auth_context = invocation_context.auth_context
    if auth_context:
        request_context["authorizer"] = auth_context
    request_context["identity"].update(invocation_context.auth_identity or {})
    if not helpers.is_test_invoke_method(method, path):
        request_context["path"] = (f"/{stage}" if stage else "") + relative_path
        request_context["stage"] = stage
    return request_context
def apply_request_response_templates(