Support default values for annotated server_fn arguments

This allows form submission with checkbox inputs to work.
For example:

    let doit = create_server_action::<DoItSFn>();
    <ActionForm action=doit>
      <input type="checkbox" name="is_good" value="true"/>
      <input type="submit"/>
    </ActionForm>

    #[server(DoItSFn, "/api")]
    pub async fn doit(#[server(default)] is_good: bool) -> Result<(), ServerFnError> {}

If is_good is absent in the request to the server API,
`Default::default()` is used instead.

server_fn_macro/src/lib.rs

@@ -84,15 +84,15 @@ pub fn server_macro_impl(
     let fn_path = fn_path.unwrap_or_else(|| Literal::string(""));
     let encoding = quote!(#server_fn_path::#encoding);
 
-    let body = syn::parse::<ServerFnBody>(body.into())?;
+    let mut body = syn::parse::<ServerFnBody>(body.into())?;
     let fn_name = &body.ident;
     let fn_name_as_str = body.ident.to_string();
     let vis = body.vis;
     let block = body.block;
 
     let fields = body
         .inputs
-        .iter()
+        .iter_mut()
         .filter(|f| {
             if let Some(ctx) = &server_context {
                 !fn_arg_is_cx(f, ctx)
@@ -110,8 +110,30 @@ pub fn server_macro_impl(
                 }
                 FnArg::Typed(t) => t,
             };
-            quote! { pub #typed_arg }
-        });
+            let mut default = false;
+            let mut other_attrs = Vec::new();
+            for attr in typed_arg.attrs.iter() {
+                if !attr.path().is_ident("server") {
+                    other_attrs.push(attr.clone());
+                    continue;
+                }
+                attr.parse_nested_meta(|meta| {
+                    if meta.path.is_ident("default") && meta.input.is_empty() {
+                        default = true;
+                        Ok(())
+                    } else {
+                        Err(meta.error("Unrecognized #[server] attribute, expected #[server(default)]"))
+                    }
+                })?;
+            }
+            typed_arg.attrs = other_attrs;
+            if default {
+                Ok(quote! { #[serde(default)] pub #typed_arg })
+            } else {
+                Ok(quote! { pub #typed_arg })
+            }
+        })
+        .collect::<Result<Vec<_>>>()?;
 
     let cx_arg = body.inputs.iter().next().and_then(|f| {
         server_context