tailcfg, ipn/ipnlocal: add Hostinfo.SSH_HostKeys, send when SSH enabled

(The name SSH_HostKeys is bad but SSHHostKeys is worse.)

Updates #3802

Change-Id: I2a889019c9e8b065b668dd58140db4fcab868a91
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f