tailcfg: pointerify RegisterRequest.Auth, omitemptify RegisterResponseAuth

We were storing server-side lots of:

    "Auth":{"Provider":"","LoginName":"","Oauth2Token":null,"AuthKey":""},

That was about 7% of our total storage of pending RegisterRequest
bodies.

Updates tailscale/corp#19327

Change-Id: Ib73842759a2b303ff5fe4c052a76baea0d68ae7d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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