Per element schema parsing in ConvertToBeamRows (#36393)

* get_bq_tableschema was invoked on every dict to row conversion if bq schema is not of tableschema type

* get_bq_tableschema was invoked on every dict to row conversion if bq schema is not of tableschema type

Author: stankiewicz

sdks/python/apache_beam/io/gcp/bigquery.py

@@ -2764,6 +2764,9 @@ def expand(self, input):
 
   class ConvertToBeamRows(PTransform):
     def __init__(self, schema, dynamic_destinations):
+      if not isinstance(schema,
+                        (bigquery.TableSchema, bigquery.TableFieldSchema)):
+        schema = bigquery_tools.get_bq_tableschema(schema)
       self.schema = schema
       self.dynamic_destinations = dynamic_destinations