### 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
pass
class ProxyListenerApiGateway(ProxyListener):
    def forward_request(self, method, path, data, headers):
        invocation_context = ApiInvocationContext(method, path, data, headers)
        forwarded_for = headers.get(HEADER_LOCALSTACK_EDGE_URL, "")
        if re.match(PATH_REGEX_USER_REQUEST, path) or "execute-api" in forwarded_for:
            result = invoke_rest_api_from_request(invocation_context)
            if result is not None:
                return result
        data = data and json.loads(to_str(data))
        if re.match(PATH_REGEX_AUTHORIZERS, path):
            return handle_authorizers(method, path, data, headers)
        if re.match(PATH_REGEX_DOC_PARTS, path):
            return handle_documentation_parts(method, path, data, headers)
        if re.match(PATH_REGEX_VALIDATORS, path):
            return handle_validators(method, path, data, headers)
        if re.match(PATH_REGEX_RESPONSES, path):
            return handle_gateway_responses(method, path, data, headers)
        if re.match(PATH_REGEX_PATH_MAPPINGS, path):
            return handle_base_path_mappings(method, path, data, headers)
        if helpers.is_test_invoke_method(method, path):