Fix potential deadlock on the GIL (#26009)

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx.pxi

@@ -45,12 +45,17 @@ cdef int _get_metadata(void *state,
   cdef size_t metadata_count
   cdef grpc_metadata *c_metadata
   def callback(metadata, grpc_status_code status, bytes error_details):
+    cdef char* c_error_details = NULL
+    if error_details is not None:
+      c_error_details = <char*> error_details
     if status == StatusCode.ok:
       _store_c_metadata(metadata, &c_metadata, &metadata_count)
-      cb(user_data, c_metadata, metadata_count, status, NULL)
+      with nogil:
+        cb(user_data, c_metadata, metadata_count, status, NULL)
       _release_c_metadata(c_metadata, metadata_count)
     else:
-      cb(user_data, NULL, 0, status, error_details)
+      with nogil:
+        cb(user_data, NULL, 0, status, c_error_details)
   args = context.service_url, context.method_name, callback,
   _spawn_callback_async(<object>state, args)
   return 0  # Asynchronous return

src/python/grpcio/grpc/_cython/_cygrpc/grpc.pxi

@@ -583,7 +583,7 @@ cdef extern from "grpc/grpc_security.h":
 
   ctypedef void (*grpc_credentials_plugin_metadata_cb)(
       void *user_data, const grpc_metadata *creds_md, size_t num_creds_md,
-      grpc_status_code status, const char *error_details)
+      grpc_status_code status, const char *error_details) nogil
 
   ctypedef struct grpc_metadata_credentials_plugin:
     int (*get_metadata)(