fix(uploads): respect db engine spec's supports_multivalues_insert value for file uploads & enable multi-insert for MSSQL (#30222)

Co-authored-by: Ville Brofeldt <33317356+villebro@users.noreply.github.com>

superset/db_engine_specs/mssql.py

@@ -53,6 +53,7 @@ class MssqlEngineSpec(BaseEngineSpec):
     max_column_name_length = 128
     allows_cte_in_subquery = False
     allow_limit_clause = False
+    supports_multivalues_insert = True
 
     _time_grain_expressions = {
         None: "{col}",