Send X-Iceberg-Access-Delegation header to signal support for vended credentials/remote signing (#436)

* Send X-Iceberg-Access-Delegation header to signal support for vended credentials/remote signing

Clients can optionally send this header to signal which delegated access pattern it can support.
At this point the iceberg-python client can support `vended-credentials` and `remote-signing`,
thus we can always send this header.
Addtional details about this header can be found in the
REST OpenAPI spec: https://github.com/apache/iceberg/blob/main/open-api/rest-catalog-open-api.yaml#L1459-L1483

* Update rest.py

Author: nastra

pyiceberg/catalog/rest.py

@@ -237,6 +237,7 @@ def _create_session(self) -> Session:
         session.headers["Content-type"] = "application/json"
         session.headers["X-Client-Version"] = ICEBERG_REST_SPEC_VERSION
         session.headers["User-Agent"] = f"PyIceberg/{__version__}"
+        session.headers["X-Iceberg-Access-Delegation"] = "vended-credentials"
 
         # Configure SigV4 Request Signing
         if str(self.properties.get(SIGV4, False)).lower() == "true":