Create rendcache.{c|h} for hidden service descriptor cache
For developers and reviewers mental sanity, it would be great to move the ABI/API of both service and client hidden service descriptor cache to its own set of files.
Furthermore, to resolve #16389 (moved), it would help a lot to do that so we can ease development, maintenance, review and have better documentation in one single file.
I propose rendcache.{c|h}.
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