tailcfg: add schema for client audit logging

updates tailscale/corp#26435

Adds the schema for generating and sendiing audit-logs to
the audit-logs endpoint.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

tailcfg/tailcfg.go

@@ -2966,3 +2966,32 @@ const LBHeader = "Ts-Lb"
 // correspond to those IPs. Any services that don't correspond to a service
 // this client is hosting can be ignored.
 type ServiceIPMappings map[ServiceName][]netip.Addr
+
+// ClientAuditAction represents an auditable action that a client can report to the
+// control plane.  These actions must correspond to the supported actions
+// in the control plane.
+type ClientAuditAction string
+
+const (
+	// AuditNodeDisconnect action must be sent when a node has disconnected
+	// from the control plane.  The details must include a reason in the Details
+	// field, either generated, or entered by the user.
+	AuditNodeDisconnect = ClientAuditAction("DISCONNECT_NODE")
+)
+
+// AuditLogRequest represents an audit log request to be sent to the control plane.
+//
+// This is JSON-encoded and sent over the control plane connection to:
+//
+//	POST https://<control-plane>/machine/audit-logs
+type AuditLogRequest struct {
+	// NodeKey is the client's current node key.
+	NodeKey key.NodePublic `json:",omitzero"`
+	// Action is the action to be logged. It must correspond to a known action in the control plane.
+	Action ClientAuditAction `json:",omitempty"`
+	// Details is an opaque string, specific to the action being logged.  Empty strings may not
+	// be valid depending on the action being logged.
+	Details string `json:",omitempty"`
+	// Timestamp is the time at which the audit log was generated on the node.
+	Timestamp time.Time `json:",omitzero"`
+}