feat(storage): expose StreamResponse content length

It can be particularly useful to know the length of the returned stream
for eg. users who want to have a progress bar for their download. Expose
it directly to avoid users needing to reach into the protected
implementation details of `self._response`.

storage/gcloud/aio/storage/storage.py

@@ -115,6 +115,10 @@ def __init__(self, response: Any) -> None:
         self._response = response
         self._iter: Optional[Iterator[bytes]] = None
 
+    @property
+    def content_length(self) -> int:
+        return int(self._response.headers.get('content-length', 0))
+
     async def read(self, size: int = -1) -> bytes:
         chunk: bytes
         if BUILD_GCLOUD_REST: