feat: check agent API version on connection (#11696)

fixes #10531

Adds a check for `version` on connection to the Agent API websocket endpoint.  This is primarily for future-proofing, so that up-level agents get a sensible error if they connect to a back-level Coderd.

It also refactors the location of the `CurrentVersion` variables, to be part of the `proto` packages, since the versions refer to the APIs defined therein.

tailnet/coordinator_test.go

@@ -460,7 +460,7 @@ func TestRemoteCoordination(t *testing.T) {
 
 	serveErr := make(chan error, 1)
 	go func() {
-		err := svc.ServeClient(ctx, tailnet.CurrentVersion.String(), sC, clientID, agentID)
+		err := svc.ServeClient(ctx, proto.CurrentVersion.String(), sC, clientID, agentID)
 		serveErr <- err
 	}()
 

tailnet/proto/version.go

@@ -0,0 +1,12 @@
+package proto
+
+import (
+	"github.com/coder/coder/v2/coderd/util/apiversion"
+)
+
+const (
+	CurrentMajor = 2
+	CurrentMinor = 0
+)
+
+var CurrentVersion = apiversion.New(CurrentMajor, CurrentMinor).WithBackwardCompat(1)

tailnet/service.go

@@ -20,13 +20,6 @@ import (
 	"golang.org/x/xerrors"
 )
 
-const (
-	CurrentMajor = 2
-	CurrentMinor = 0
-)
-
-var CurrentVersion = apiversion.New(CurrentMajor, CurrentMinor).WithBackwardCompat(1)
-
 type streamIDContextKey struct{}
 
 // StreamID identifies the caller of the CoordinateTailnet RPC.  We store this