added alter table command to fluent query builder

include/matador/sql/query_context.hpp

@@ -10,6 +10,7 @@ namespace matador::sql {
 
 enum class sql_command {
     SQL_UNKNOWN,
+    SQL_CREATE,
     SQL_CREATE_TABLE,
     SQL_CREATE_SCHEMA,
     SQL_CREATE_DATABASE,
@@ -17,6 +18,7 @@ enum class sql_command {
     SQL_INSERT,
     SQL_DELETE,
     SQL_SELECT,
+    SQL_DROP,
     SQL_DROP_TABLE,
     SQL_DROP_SCHEMA,
     SQL_DROP_DATABASE,