Generify AggregatorFactory and PostAggregator

[leventov]

If the generic parameter is called T, the return type of compute() should be T and the return type of getComparator() should be Comparator<? super T>.

Although PostAggregator is an ExtensionPoint this should be a compatible change, because generics in Java were designed to support compatible transition.

[gianm]

Maybe do the same for AggregatorFactory.combine and AggregatorFactory.getComparator.

[stale]

This issue has been marked as stale due to 280 days of inactivity. It will be closed in 2 weeks if no further activity occurs. If this issue is still relevant, please simply write any comment. Even if closed, you can still revive the issue at any time or discuss it on the dev@druid.apache.org list. Thank you for your contributions.

[stale]

This issue has been closed due to lack of activity. If you think that is incorrect, or the issue requires additional review, you can revive the issue at any time.

[github-actions]

This issue has been marked as stale due to 280 days of inactivity.
It will be closed in 4 weeks if no further activity occurs. If this issue is still
relevant, please simply write any comment. Even if closed, you can still revive the
issue at any time or discuss it on the dev@druid.apache.org list.
Thank you for your contributions.

[github-actions]

This issue has been closed due to lack of activity. If you think that
is incorrect, or the issue requires additional review, you can revive the issue at
any time.