Add template for socket_filter program type

This patch adds template for socket_filter.
e.g.

```sh
cargo generate --path ~/dev/aya-template \
         --name my-test \
	 -d program_type=socket_filter
```

.github/workflows/ci.yml

@@ -26,6 +26,7 @@ jobs:
           - uprobe
           - uretprobe
           - sock_ops
+          - sock_filter
           - sk_msg
           - xdp
           - classifier

cargo-generate.toml

@@ -13,6 +13,7 @@ choices = [
     "uprobe",
     "uretprobe",
     "sock_ops",
+    "sock_filter",
     "sk_msg",
     "xdp",
     "classifier",

{{project-name}}-ebpf/src/main.rs

@@ -245,6 +245,16 @@ pub fn {{tracepoint_name}}(ctx: BtfTracePointContext) -> i32 {
 unsafe fn try_{{tracepoint_name}}(_ctx: BtfTracePointContext) -> Result<i32, i32> {
     Ok(0)
 }
+{%- when "socket_filter" %}
+use aya_bpf::{
+    macros::socket_filter,
+    programs::SkBuffContext,
+};
+
+#[socket_filter(name="{{crate_name}}")]
+pub fn {{crate_name}}(_ctx: SkBuffContext) -> i64 {
+    return 0
+}
 {%- endcase %}
 
 #[panic_handler]

{{project-name}}/src/main.rs

@@ -28,6 +28,10 @@ use aya::programs::TracePoint;
 use aya::{programs::Lsm, Btf};
 {%- when "tp_btf" -%}
 use aya::{programs::BtfTracePoint, Btf};
+{%- when "socket_filter" -%}
+use std::net::TcpStream;
+use std::os::unix::io::AsRawFd;
+use aya::programs::SocketFilter;
 {%- endcase %}
 use log::info;
 use simplelog::{ColorChoice, ConfigBuilder, LevelFilter, TermLogger, TerminalMode};
@@ -131,6 +135,11 @@ async fn main() -> Result<(), anyhow::Error> {
     let program: &mut BtfTracePoint = bpf.program_mut("{{tracepoint_name}}").unwrap().try_into()?;
     program.load("{{tracepoint_name}}", &btf)?;
     program.attach()?;
+    {%- when "socket_filter" -%}
+    let client = TcpStream::connect("127.0.0.1:1234")?;
+    let prog: &mut SocketFilter = bpf.program_mut("{{crate_name}}").unwrap().try_into()?;
+    prog.load()?;
+    prog.attach(client.as_raw_fd())?;
     {%- endcase %}
 
     info!("Waiting for Ctrl-C...");