offer function-level control over tracing (#5194)

* wiggle: fix compilation with async functions when tracing is off

Fixes #5202

* switch tracing config from a boolean to a struct

This will enable more complex tracing rules in the future

* rename AsyncConfField to FunctionField

It is going to be reused for cases other than just async functions

* add support for disabling tracing per-function

This adds a `disable_for` syntax after the `tracing` boolean.  For
example:

```
wiggle::from_witx!(
    tracing: true disable_for {
        module1::foo,
        module2::{bar, baz},
    }
)
```

crates/wiggle/generate/src/funcs.rs

@@ -84,7 +84,7 @@ fn _define_func(
         );
     );
     if settings.get_async(&module, &func).is_sync() {
-        let traced_body = if settings.tracing {
+        let traced_body = if settings.tracing.enabled_for(&mod_name, &func_name) {
             quote!(
                 #mk_span
                 _span.in_scope(|| {
@@ -109,7 +109,7 @@ fn _define_func(
             bounds,
         )
     } else {
-        let traced_body = if settings.tracing {
+        let traced_body = if settings.tracing.enabled_for(&mod_name, &func_name) {
             quote!(
                 use #rt::tracing::Instrument as _;
                 #mk_span
@@ -261,7 +261,12 @@ impl witx::Bindgen for Rust<'_> {
                         args.push(quote!(#name));
                     }
                 }
-                if self.settings.tracing && func.params.len() > 0 {
+                if self
+                    .settings
+                    .tracing
+                    .enabled_for(self.module.name.as_str(), self.funcname)
+                    && func.params.len() > 0
+                {
                     let args = func
                         .params
                         .iter()
@@ -290,7 +295,11 @@ impl witx::Bindgen for Rust<'_> {
                         let ret = #trait_name::#ident(ctx, #(#args),*).await;
                     })
                 };
-                if self.settings.tracing {
+                if self
+                    .settings
+                    .tracing
+                    .enabled_for(self.module.name.as_str(), self.funcname)
+                {
                     self.src.extend(quote! {
                         #rt::tracing::event!(
                             #rt::tracing::Level::TRACE,