tailcfg: add tailnet field to register request (#9675)

Updates tailscale/corp#10967

Signed-off-by: Claire Wang <claire@tailscale.com>

tailcfg/tailcfg.go

@@ -1100,6 +1100,17 @@ type RegisterRequest struct {
 	Timestamp     *time.Time    `json:",omitempty"` // creation time of request to prevent replay
 	DeviceCert    []byte        `json:",omitempty"` // X.509 certificate for client device
 	Signature     []byte        `json:",omitempty"` // as described by SignatureType
+
+	// Tailnet is an optional identifier specifying the name of the recommended or required
+	// network that the node should join. Its exact form should not be depended on; new
+	// forms are coming later. The identifier is generally a domain name (for an organization)
+	// or e-mail address (for a personal account on a shared e-mail provider). It is the same name
+	// used by the API, as described in /api.md#tailnet.
+	// If Tailnet begins with the prefix "required:" then the server should prevent logging in to a different
+	// network than the one specified. Otherwise, the server should recommend the specified network
+	// but still permit logging in to other networks.
+	// If empty, no recommendation is offered to the server and the login page should show all options.
+	Tailnet string `json:",omitempty"`
 }
 
 // RegisterResponse is returned by the server in response to a RegisterRequest.

tailcfg/tailcfg_clone.go

@@ -362,6 +362,7 @@ var _RegisterRequestCloneNeedsRegeneration = RegisterRequest(struct {
 	Timestamp        *time.Time
 	DeviceCert       []byte
 	Signature        []byte
+	Tailnet          string
 }{})
 
 // Clone makes a deep copy of DERPHomeParams.

tailcfg/tailcfg_view.go

@@ -792,6 +792,7 @@ func (v RegisterRequestView) DeviceCert() views.ByteSlice[[]byte] {
 func (v RegisterRequestView) Signature() views.ByteSlice[[]byte] {
 	return views.ByteSliceOf(v.ж.Signature)
 }
+func (v RegisterRequestView) Tailnet() string { return v.ж.Tailnet }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _RegisterRequestViewNeedsRegeneration = RegisterRequest(struct {
@@ -810,6 +811,7 @@ var _RegisterRequestViewNeedsRegeneration = RegisterRequest(struct {
 	Timestamp        *time.Time
 	DeviceCert       []byte
 	Signature        []byte
+	Tailnet          string
 }{})
 
 // View returns a readonly view of DERPHomeParams.