control/controlclient: send load balancing hint HTTP request header

Updates tailscale/corp#1297

Change-Id: I0b102081e81dfc1261f4b05521ab248a2e4a1298
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