### 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[HEADER_TARGET_API] = str(api)
        headers["Host"] = host
        if isinstance(data, dict):
            data = json.dumps(data)
        encoding_type = headers.get("Content-Encoding") or ""
        if encoding_type.upper() == GZIP_ENCODING.upper() and api not in SKIP_GZIP_APIS:
            headers.set("Content-Encoding", IDENTITY_ENCODING)
            data = gzip.decompress(data)
        is_internal_call = is_internal_call_context(headers)
        self._require_service(api)
        lock_ctx = BOOTSTRAP_LOCK
        if is_internal_call or persistence.is_persistence_restored():
            lock_ctx = empty_context_manager()
        with lock_ctx:
            result = do_forward_request(api, method, path, data, headers, port=port)
            if should_log_trace and result not in [None, False, True]:
                result_status_code = getattr(result, "status_code", result)
                result_headers = getattr(result, "headers", {})
                result_content = getattr(result, "content", "")
                LOG.debug(
                    'OUT(%s): "%s %s" - status: %s - response headers: %s - response: %s',
                    api,
                    method,
                    path,
                    result_status_code,
                    dict(result_headers or {}),
                    result_content,
                )
            return result
    def return_response(self, method, path, data, headers, response):