Add templated types to getSegmentsByClass and getFirstSegmentInstanceByClass

[jay-knight]

Consider code like

$ORC = $message->getFirstSegmentInstanceByClass(ORC::class);
$ORC->getOrderControl();

PHPStan will complain that Call to an undefined method Aranyasen\HL7\Segment::getOrderControl(). This is because that function declares that it will return a Segment object, but it doesn't specify which subclass of Segment, so it doesn't know to look in the ORC class for the method. Using the @template phpdoc annotation, you can specify that the return type will be the same as the class string you pass in.

This PR adds this, which resolves a lot of PHPstan errors in our codebase.