Python: Set the Iceberg Spec version (apache#5766)

We pass in the version of the response that we expect for a certain
version. If we change anything in the future in the spec, we can
maintain backward compatibility until the version is being bumped.

python/pyiceberg/catalog/rest.py

@@ -58,6 +58,8 @@
 from pyiceberg.typedef import EMPTY_DICT
 from pyiceberg.utils.iceberg_base_model import IcebergBaseModel
 
+ICEBERG_REST_SPEC_VERSION = "0.14.1"
+
 
 class Endpoints:
     get_config: str = "config"
@@ -195,7 +197,8 @@ def _check_valid_namespace_identifier(self, identifier: Union[str, Identifier])
     def headers(self) -> Properties:
         headers = {
             "Content-type": "application/json",
-            "X-Client-Version": __version__,
+            "X-Client-Version": ICEBERG_REST_SPEC_VERSION,
+            "User-Agent": f"PyIceberg/{__version__}",
         }
         if token := self.properties.get("token"):
             headers[AUTHORIZATION_HEADER] = f"{BEARER_PREFIX} {token}"