So derpers can check an external URL for whether to permit access
to a certain public key.
Updates tailscale/corp#17693
Change-Id: I8594de58f54a08be3e2dbef8bcd1ff9b728ab297
Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>