x/staking: add ValidateGenesis benchmark

[odeke-em]

This benchmark examines how ValidateGenesis behaves.
In a follow-up issue, I'll show how it reveals an inefficiency
that'll affect trying to load repeatedly retrieve Validators'
ConsAddress values.

Updates #8744

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[tac0turtle]

LGTM.

Are you working on benchmarking other parts of staking?

[odeke-em]

Thanks for the review @marbar3778!

Are you working on benchmarking other parts of staking?

Kind of, my initial priority is getting objective benchmarks and comparisons that anyone can see improvements on, and a comparison mechanism. To do this I investigate various parts of the SDK that are slow and then when I am going to suggest a change, send a benchmark.