Add an option to sort sponsors by the length of time a user has been sponsored

[JohnTitor]

Clear and concise description of the problem

Currently, sponsorkit only allows sorting sponsors by sponsoring tiers.
That's great for some cases, but I'd like to sort them by the length of time a user has been a sponsor, like over 1 year, 6 months, 3 months, etc..., because continuous support is also an important thing for FLOSS maintainers, at least I believe so.
(Someone also may not want to reveal sponsors' tiers explicitly.)

Suggested solution

The current implementation is tied to the sponsoring tiers so some tweaks to the config would be needed as well as sorting implementation.
I've implemented this in a quite dirty way (replacing tiers directly instead of adding an option): JohnTitor@f7d7b13
(The above way works fine for me, but it'd be great if upstream supported this use case so that I don't have to worry about my fork :))

Alternative

No response

Additional context

Thank you for the wonderful tool by the way!

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