feat: add AgentAPI using DRPC (#10811)

Co-authored-by: Spike Curtis <spike@coder.com>

coderd/agentapi/tailnet.go

@@ -0,0 +1,53 @@
+package agentapi
+
+import (
+	"context"
+	"time"
+
+	"golang.org/x/xerrors"
+	"tailscale.com/tailcfg"
+
+	agentproto "github.com/coder/coder/v2/agent/proto"
+	"github.com/coder/coder/v2/tailnet"
+	tailnetproto "github.com/coder/coder/v2/tailnet/proto"
+)
+
+type TailnetAPI struct {
+	Ctx                    context.Context
+	DerpMapFn              func() *tailcfg.DERPMap
+	DerpMapUpdateFrequency time.Duration
+}
+
+func (a *TailnetAPI) StreamDERPMaps(_ *tailnetproto.StreamDERPMapsRequest, stream agentproto.DRPCAgent_StreamDERPMapsStream) error {
+	defer stream.Close()
+
+	ticker := time.NewTicker(a.DerpMapUpdateFrequency)
+	defer ticker.Stop()
+
+	var lastDERPMap *tailcfg.DERPMap
+	for {
+		derpMap := a.DerpMapFn()
+		if lastDERPMap == nil || !tailnet.CompareDERPMaps(lastDERPMap, derpMap) {
+			protoDERPMap := tailnet.DERPMapToProto(derpMap)
+			err := stream.Send(protoDERPMap)
+			if err != nil {
+				return xerrors.Errorf("send derp map: %w", err)
+			}
+			lastDERPMap = derpMap
+		}
+
+		ticker.Reset(a.DerpMapUpdateFrequency)
+		select {
+		case <-stream.Context().Done():
+			return nil
+		case <-a.Ctx.Done():
+			return nil
+		case <-ticker.C:
+		}
+	}
+}
+
+func (*TailnetAPI) CoordinateTailnet(_ agentproto.DRPCAgent_CoordinateTailnetStream) error {
+	// TODO: implement this
+	return xerrors.New("CoordinateTailnet is unimplemented")
+}