feat: The Ahlswede-Zhang identity (#8171)

The Ahlswede-Zhang identity is a sharpening of the [Lubell-Yamamoto-Meshalkin inequality](https://leanprover-community.github.io/mathlib_docs/combinatorics/set_family/lym.html#finset.sum_card_slice_div_choose_le_one), by expliciting the correction term.

This PR defines `Finset.truncatedSup`/`Finset.truncatedInf`, whose cardinalities show up in the correction term, and subsequently proves the Ahlswede-Zhang identity itself.

Co-authored-by: Vladimir Ivanov <volodimir1024@gmail.com>

Mathlib.lean

@@ -1355,6 +1355,7 @@ import Mathlib.Combinatorics.Quiver.SingleObj
 import Mathlib.Combinatorics.Quiver.Subquiver
 import Mathlib.Combinatorics.Quiver.Symmetric
 import Mathlib.Combinatorics.Schnirelmann
+import Mathlib.Combinatorics.SetFamily.AhlswedeZhang
 import Mathlib.Combinatorics.SetFamily.CauchyDavenport
 import Mathlib.Combinatorics.SetFamily.Compression.Down
 import Mathlib.Combinatorics.SetFamily.Compression.UV