net/udprelay{/endpoint}, all: move ServerEndpoint to independent pkg (#15934)

ServerEndpoint will be used within magicsock and potentially elsewhere,
which should be possible without needing to import the server
implementation itself.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/endpoint/endpoint.go

@@ -0,0 +1,55 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package endpoint contains types relating to UDP relay server endpoints. It
+// does not import tailscale.com/net/udprelay.
+package endpoint
+
+import (
+	"net/netip"
+
+	"tailscale.com/tstime"
+	"tailscale.com/types/key"
+)
+
+// ServerEndpoint contains details for an endpoint served by a
+// [tailscale.com/net/udprelay.Server].
+type ServerEndpoint struct {
+	// ServerDisco is the Server's Disco public key used as part of the 3-way
+	// bind handshake. Server will use the same ServerDisco for its lifetime.
+	// ServerDisco value in combination with LamportID value represents a
+	// unique ServerEndpoint allocation.
+	ServerDisco key.DiscoPublic
+
+	// LamportID is unique and monotonically non-decreasing across
+	// ServerEndpoint allocations for the lifetime of Server. It enables clients
+	// to dedup and resolve allocation event order. Clients may race to allocate
+	// on the same Server, and signal ServerEndpoint details via alternative
+	// channels, e.g. DERP. Additionally, Server.AllocateEndpoint() requests may
+	// not result in a new allocation depending on existing server-side endpoint
+	// state. Therefore, where clients have local, existing state that contains
+	// ServerDisco and LamportID values matching a newly learned endpoint, these
+	// can be considered one and the same. If ServerDisco is equal, but
+	// LamportID is unequal, LamportID comparison determines which
+	// ServerEndpoint was allocated most recently.
+	LamportID uint64
+
+	// AddrPorts are the IP:Port candidate pairs the Server may be reachable
+	// over.
+	AddrPorts []netip.AddrPort
+
+	// VNI (Virtual Network Identifier) is the Geneve header VNI the Server
+	// will use for transmitted packets, and expects for received packets
+	// associated with this endpoint.
+	VNI uint32
+
+	// BindLifetime is amount of time post-allocation the Server will consider
+	// the endpoint active while it has yet to be bound via 3-way bind handshake
+	// from both client parties.
+	BindLifetime tstime.GoDuration
+
+	// SteadyStateLifetime is the amount of time post 3-way bind handshake from
+	// both client parties the Server will consider the endpoint active lacking
+	// bidirectional data flow.
+	SteadyStateLifetime tstime.GoDuration
+}