Add map library with merge functionality

[gfrey]

Allows to merge two maps. Maps must have identical value types. Keys of the second map take precedence.

I initially wrote this library for kro (see kubernetes-sigs/kro#628), but was asked to contribute it here.

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[TristonianJones]

/gcbrun

[a-hilaly]

Hey @TristonianJones - this PR was originally opened against kubernetes-sigs/kro code base and we advised to contribute to upstream cel-go instead. Do you think this is something you're interested in taking? :)

[TristonianJones]

@a-hilaly thanks for the referral. I think we just need to consider how best to make this approach work for a variety of types and a variety of merge heuristics. Definitely interested though.