feat: add additional column ordering to keysetpagination

[jonas-jonas]

cc @zepatrik

Related Issue or Design Document

Checklist

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• I have referenced an issue containing the design document if my change introduces a new feature.
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Further comments

[zepatrik]

Nice, this already looks very good 👍
Should we maybe allow multiple additional fields? We might not use it, but then it will be easy to do once it comes up.

[jonas-jonas]

@zepatrik I'd like to postpone the support for multiple fields for now. I've spent too much time on this already, and that would just complicate it, IMO. I'll add some more tests, but then it should be good to go.

[zepatrik]

Suggested change

	type PageTokenConstructor func(string) (PageToken, error)
	type PageTokenConstructor = func(string) (PageToken, error)

A type alias allows to just pass a value without ever needing to do type assertions.

[jonas-jonas]

Ah, nice. Don't think this changes much in the current PR. Or did you have something specific in mind?

[zepatrik]

Nice 🎉