cmd/tailscale, ipn/ipnlocal: add debug command to write to StateStore for dev

Not for end users (unless directed by support). Mostly for ease of
development for some upcoming webserver work.

Change-Id: I43acfed217514567acb3312367b24d620e739f88
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f