feat(eBPF): embed BTF information in eBPF kernel (#8842)

It turns out that the Rust compiler doesn't always say that it is adding
debug information to a binary even when it does! The build output only
displays `[optimized]` when in fact it does actually emit debug
information. Adding an additional linker flag configures `bpf-linker` to
include the necessary BTF information in our kernel.

This makes debugging verifier errors much easier as the program output
contains source code annotiations. It also should make it easier to
debug issues using `xdpdump` which relies on BTF information.

Resolves: #8503

rust/Cargo.toml

@@ -234,3 +234,6 @@ split-debuginfo = "packed"
 [profile.release.package.firezone-gui-client]
 debug = "full"
 split-debuginfo = "packed"
+
+[profile.release.package.ebpf-turn-router]
+debug = 2

rust/relay/ebpf-turn-router/build.rs

@@ -14,4 +14,11 @@ use which::which;
 fn main() {
     let bpf_linker = which("bpf-linker").expect("bpf-linker not found in $PATH");
     println!("cargo:rerun-if-changed={}", bpf_linker.to_str().unwrap());
+
+    if std::env::var("CARGO_BUILD_TARGET")
+        .ok()
+        .is_some_and(|t| t == "bpfel-unknown-none" || t == "bpfeb-unknown-unknown")
+    {
+        println!("cargo:rustc-link-arg=--btf");
+    }
 }