fix(parser): Limit named PK parsing to MySQL only

[VaggelisD]

Fixes #6989

The regression is due to #6389 as it tried to parse MySQL's named primary keys such as PRIMARY KEY name (...), which now trips up for Clickhouse's PRIMARY KEY tuple (...).

I don't think named primary keys exist outside of MySQL (it itself seems to discard the name anyways) so I opted out of using a parser flag to generalize the behavior.

# Naming the PK does not work
:) CREATE TABLE test_table (id Int32, name String) ENGINE=MergeTree PRIMARY KEY "foo" (id);
Code: 46. DB::Exception: Unknown function foo: While processing foo(id). (UNKNOWN_FUNCTION)

# tuple() works
:) CREATE TABLE test_table (id Int32, name String) ENGINE=MergeTree PRIMARY KEY tuple();
OK

# wrapped tuple() also works
:) CREATE TABLE test_table (id Int32, name String) ENGINE=MergeTree PRIMARY KEY (tuple());
OK

Docs

Clickhouse PK