Add HID-BPF tutorial and implementation for virtual mouse input modification

- Introduced a comprehensive tutorial in README.md explaining how to fix broken HID devices using eBPF without kernel patches.
- Implemented a userspace program (hid-input-modifier.c) that creates a virtual HID mouse using the uhid interface and sends synthetic mouse events.
- Developed a BPF program (hid-input-modifier.bpf.c) that intercepts HID events and modifies mouse movement data, effectively doubling the X and Y movement.
- Created necessary header files (hid_bpf.h, hid_bpf_defs.h, hid_bpf_helpers.h) to define structures and helper functions for the BPF program.
- Added functionality to find and manage the virtual HID device, ensuring seamless integration with the BPF program.

src/43-kfuncs/README.md

@@ -2,6 +2,8 @@
 
 Have you ever felt constrained by eBPF's capabilities? Maybe you've run into situations where the existing eBPF features just aren't enough to accomplish your goals. Perhaps you need deeper interactions with the kernel, or you're facing performance issues that the standard eBPF runtime can't solve. If you've ever wished for more flexibility and power in your eBPF programs, this tutorial is for you.
 
+> The complete source code: <https://github.com/eunomia-bpf/bpf-developer-tutorial/tree/main/src/43-kfuncs>
+
 ## Introduction: Adding a `strstr` kfunc to Break Free from eBPF Runtime Limitations
 
 **eBPF (extended Berkeley Packet Filter)** has revolutionized Linux system programming by allowing developers to run sandboxed programs inside the kernel. It's a game-changer for networking, security, and observability, enabling powerful functionalities without the need to modify kernel source code or load traditional kernel modules.