### 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
LOCALSTACK_VENV_FOLDER = os.path.join(LOCALSTACK_ROOT_FOLDER, ".venv")
    if not os.path.isdir(LOCALSTACK_VENV_FOLDER):
        
        LOCALSTACK_VENV_FOLDER = os.path.realpath(
            os.path.join(LOCALSTACK_ROOT_FOLDER, "..", "..", "..")
        )
PATH_USER_REQUEST = "_user_request_"
DOCKER_IMAGE_NAME = "localstack/localstack"
DOCKER_IMAGE_NAME_FULL = "localstack/localstack-full"
CONFIG_UPDATE_PATH = "/?_config_"
INTERNAL_RESOURCE_PATH = "/_localstack"
ENV_INTERNAL_TEST_RUN = "LOCALSTACK_INTERNAL_TEST_RUN"
ENV_PRO_ACTIVATED = "PRO_ACTIVATED"
HEADER_CONTENT_TYPE = "Content-Type"
APPLICATION_AMZ_JSON_1_0 = "application/x-amz-json-1.0"
APPLICATION_AMZ_JSON_1_1 = "application/x-amz-json-1.1"
APPLICATION_AMZ_CBOR_1_1 = "application/x-amz-cbor-1.1"
APPLICATION_CBOR = "application/cbor"
APPLICATION_JSON = "application/json"
APPLICATION_XML = "application/xml"
APPLICATION_OCTET_STREAM = "application/octet-stream"
APPLICATION_X_WWW_FORM_URLENCODED = "application/x-www-form-urlencoded"
HEADER_ACCEPT_ENCODING = "Accept-Encoding"
TRUE_STRINGS = ("1", "true", "True")
FALSE_STRINGS = ("0", "false", "False")
LOG_LEVELS = ("trace-internal", "trace", "debug", "info", "warn", "error", "warning")
LAMBDA_TEST_ROLE = "arn:aws:iam::%s:role/lambda-test-role" % TEST_AWS_ACCOUNT_ID
ELASTICSEARCH_DEFAULT_VERSION = "Elasticsearch_7.10"
ELASTICSEARCH_PLUGIN_LIST = [
    "analysis-icu",