Join class does not support joining tables by non-id fields

[DarkSide666]

Currently we do not support joining tables by non-id fields because we don't know how to save records in such case.

See #802 and "Joining tables on non-id fields is not implemented yet" exception in Join class.

[mvorisek]

here is a repro code:

data/tests/JoinSqlTest.php

Lines 32 to 36 in f89264b

	$this->expectException(Exception::class); // TODO not implemented yet, see https://github.com/atk4/data/issues/803
	$j = $m->join('contact4.foo_id', ['master_field' => 'test_id', 'reverse' => true]);
	// $this->assertTrue($this->getProtected($j, 'reverse'));
	// $this->assertSame('test_id', $this->getProtected($j, 'master_field'));
	// $this->assertSame('foo_id', $this->getProtected($j, 'foreign_field'));

[mvorisek]

Currently there are these problems:

• Join::reverse = true with join on non-ID field is not supported (Joining tables on non-id fields is not implemented yet exception is thrown)
• foreign ID field type is hardcoded to integer
• join ID field type is hardcoded to integer