TUN-8861: Add session limiter to UDP session manager

## Summary
In order to make cloudflared behavior more predictable and
prevent an exhaustion of resources, we have decided to add
session limits that can be configured by the user. This first
commit introduces the session limiter and adds it to the UDP
handling path. For now the limiter is set to run only in
unlimited mode.

vendor/go.uber.org/mock/gomock/string.go

@@ -0,0 +1,36 @@
+package gomock
+
+import (
+	"fmt"
+	"reflect"
+)
+
+// getString is a safe way to convert a value to a string for printing results
+// If the value is a a mock, getString avoids calling the mocked String() method,
+// which avoids potential deadlocks
+func getString(x any) string {
+	if isGeneratedMock(x) {
+		return fmt.Sprintf("%T", x)
+	}
+	if s, ok := x.(fmt.Stringer); ok {
+		return s.String()
+	}
+	return fmt.Sprintf("%v", x)
+}
+
+// isGeneratedMock checks if the given type has a "isgomock" field,
+// indicating it is a generated mock.
+func isGeneratedMock(x any) bool {
+	typ := reflect.TypeOf(x)
+	if typ == nil {
+		return false
+	}
+	if typ.Kind() == reflect.Ptr {
+		typ = typ.Elem()
+	}
+	if typ.Kind() != reflect.Struct {
+		return false
+	}
+	_, isgomock := typ.FieldByName("isgomock")
+	return isgomock
+}