ENH Add generic types

[GuySartorelli]

Description

• Add generic types where appropriate, e.g. for indicating the type of records inside DataList or ArrayList.
• Replace @property annotation for extension $owner property with a generic extension. This works effectively the same way, with the bonus benefit that the return type for getOwner() is also correct.
• Remove unnecessary @var comments.

Requires silverstripe/silverstripe-framework#11108 for these generics to take effect.

Manual testing steps

You can validate that the types are evaluating correctly in your preferred code editor.

Issues

• Add generic typehints in PHPDoc silverstripe/silverstripe-framework#11066

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[tractorcow]

Nice work, I love attention to phpdoc.