Add an optional verifier to crowdloan (#2248)

* Add an optional verifier to crowdloan

* add tests

* verify signatures

* benchmark with signatures

* update crowdloan benchmark

* try to get keystore working in test

* rewrite to avoid traits

* Use MultiSignature and MultiSigner

* refactor and update benchmarks

* optimize check order

* fix no_std build

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
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