feat(api): support a list of functions to apply across a selector

[cpcloud]

Is your feature request related to a problem?

No, it is not.

Describe the solution you'd like

I'd like to be able to write something similar to this:

t.agg(s.across(my_selector, ["max", "mean", "min"]))

and have that be equivalent to

t.agg(s.across(my_selector, dict(max=_.max(), mean=_.mean(), min=_.min())))

which we already support.

What version of ibis are you running?

main

What backend(s) are you using, if any?

No response

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