tailcfg: add CapabilityDataPlaneAuditLogs.

We're going to want to enable audit logging on a per-Tailnet basis. When this
happens, we want control to inform the Tailnet's clients of this capability.

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

tailcfg/tailcfg.go

@@ -1530,6 +1530,7 @@ const (
 	CapabilityAdmin              = "https://tailscale.com/cap/is-admin"
 	CapabilitySSH                = "https://tailscale.com/cap/ssh"                   // feature enabled/available
 	CapabilitySSHRuleIn          = "https://tailscale.com/cap/ssh-rule-in"           // some SSH rule reach this node
+	CapabilityDataPlaneAuditLogs = "https://tailscale.com/cap/data-plane-audit-logs" // feature enabled
 
 	// These are the capabilities that the peer nodes have as listed in
 	// MapResponse.Peers[].Capabilities.