Extract net9 alternate-key cache support for ConcurrentLru#739

Copilot wants to merge 9 commits intomainbitfaster/BitFaster.Caching:mainfrom

copilot/add-alternate-cache-from-concurrentlrubitfaster/BitFaster.Caching:copilot/add-alternate-cache-from-concurrentlruCopy head branch name to clipboard