ENH Add generic types - Githubissues

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.

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

[x] The target branch is correct
- See picking the right version
[x] All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
- Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
[x] The commit messages follow our commit message guidelines
[x] The PR follows our contribution guidelines
[x] Code changes follow our coding conventions
[x] Tests aren't necessary for this change
[x] CI is green