### 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
headers = {} if response is None else response.headers
    
    
    
    if isinstance(response, LambdaResponse):
        for key in response.multi_value_headers.keys():
            headers_list = list(response.multi_value_headers[key]) + [response.headers.get(key)]
            headers_list = [str(h) for h in headers_list if h is not None]
            headers[key] = ",".join(headers_list)
        response.multi_value_headers = {}
    if ACL_ORIGIN not in headers:
        headers[ACL_ORIGIN] = (
            request_headers["origin"]
            if request_headers.get("origin") and not config.DISABLE_CORS_CHECKS
            else "*"
        )
    if ACL_METHODS not in headers:
        headers[ACL_METHODS] = ",".join(CORS_ALLOWED_METHODS)
    if ACL_ALLOW_HEADERS not in headers:
        requested_headers = headers.get(ACL_REQUEST_HEADERS, "")
        requested_headers = re.split(r"[,\s]+", requested_headers) + CORS_ALLOWED_HEADERS
        headers[ACL_ALLOW_HEADERS] = ",".join([h for h in requested_headers if h])
    if ACL_EXPOSE_HEADERS not in headers:
        headers[ACL_EXPOSE_HEADERS] = ",".join(CORS_EXPOSE_HEADERS)
    for header in ALLOWED_CORS_RESPONSE_HEADERS:
        if headers.get(header) == "":
            del headers[header]
def http_exception_to_response(e: HTTPException):