Make the microdesc format a little more flexible, with optional TAP key
Right now we say that a microdesc starts with an onion-key field, and the onion-key contains a 1024-bit RSA key. This is true of any microdesc that we can currently use, but for caching purposes, we should probably be more tolerant so that we can have caches tolerate other key types in the distant happy future.
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