alter table <table> add column with multiple columns not supported

Example query:

create table t (pk int primary key);
alter table t add column (col1 int, col2 int);
desc t;

MySql output:

Field	Type	Null	Key	Default	Extra
pk	int	NO	PRI	NULL	
col1	int	YES		NULL	
col2	int	YES		NULL

Our output:

Error parsing SQL
syntax error at position 36 near 'int'
alter table t add column (col1 int, col2 int)
                                    ^

The offending rule in sql.y:

alter_table_statement_part:
  ADD column_opt '(' column_definition ')'

Vitess parses the parentheses but expects only a single element within them.

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