tka: implement AUM and Key types

This is the first in a series of PRs implementing the internals for the
Tailnet Key Authority. This PR implements the AUM and Key types, which
are used by pretty much everything else. Future PRs:

 - The State type & related machinery
 - The Tailchonk (storage) type & implementation
 - The Authority type and sync implementation

Signed-off-by: Tom DNetto <tom@tailscale.com>
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