Support for get_pk_constraint() method

[akurdyukov]

Describe the bug
Currently driver does not support get_pk_constraint() method. This prevents me from working meltano tap.

To Reproduce
The test I wrote looks like

    @require_server_version(18, 16, 0)
    def test_primary_keys(self):
        self.table_with_primary.create(self.session.bind)
        con = self.dialect.get_pk_constraint(self.session,
                                             self.table_with_primary.name)
        self.assertIsNotNone(con,
                             "Table should contain primary key constrain")
        self.assertTrue("constrained_columns" in con)
        self.assertEqual(("x",), con["constrained_columns"])

where table_with_primary is

        self.table_with_primary = Table(
            'test_primary_table',
            self.metadata(),
            Column('x', types.Int32, primary_key=True),
            engines.MergeTree(
                primary_key=['x']
            )
        )

Expected behavior
Test should pass.

Versions

• Current main branch
• Any Python version