ipn/ipnlocal: add c2n method to get SSH username candidates

For control to fetch a list of Tailscale SSH username candidates to
filter against the Tailnet's SSH policy to present some valid
candidates to a user.

Updates #3802
Updates tailscale/corp#7007

Change-Id: I3dce57b7a35e66891d5e5572e13ae6ef3c898498
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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