tailcfg,ipn,appc: add c2n endpoint for appc domain routes

This change introduces a c2n endpoint that returns a map of domains to a
slice of resolved IP addresses for the domain.

Fixes tailscale/corp#15657

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

tailcfg/c2ntypes.go

@@ -5,6 +5,8 @@
 
 package tailcfg
 
+import "net/netip"
+
 // C2NSSHUsernamesRequest is the request for the /ssh/usernames.
 // A GET request without a request body is equivalent to the zero value of this type.
 // Otherwise, a POST request with a JSON-encoded request body is expected.
@@ -64,3 +66,12 @@ type C2NPostureIdentityResponse struct {
 	// device posture collection.
 	PostureDisabled bool `json:",omitempty"`
 }
+
+// C2NAppConnectorDomainRoutesResponse contains a map of domains to
+// slice of addresses, indicating what IP addresses have been resolved
+// for each domain.
+type C2NAppConnectorDomainRoutesResponse struct {
+	// Domains is a map of lower case domain names with no trailing dot,
+	// to a list of resolved IP addresses.
+	Domains map[string][]netip.Addr
+}