Make TAP keys optional when parsing documents.

Nick Mathewsonrequested to merge
nickm/arti:tap-out-phase-1 into main

This is the client-side part of phase 1 for proposal 350, which will eventually remove TAP completely from the Tor network.

Please don't merge unless @dgoulet has approved and merged torspec!268 (merged).

Edited by Nick Mathewson

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