Raw byte accessors for key types, mirroring existing patterns
(NodePublic.Raw32 and DiscoPublicFromRaw32 already exist).
NodePrivate.Raw32 returns the raw 32 bytes of a node private key.
DiscoPrivateFromRaw32 parses a 32-byte raw value as a DiscoPrivate.
Updates tailscale/corp#24454
Change-Id: Ibc08bed14ab359eddefbebd811c375b6365c7919
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
525f7a1e47