[vm/ffi] Allow omitting `@Native` type if it can be inferred from the Dart signature

[dcharkes]

On struct fields, we don't require annotating the native types if they are identical to the Dart type. This is true for Pointer, and subtypes of Struct and Union. In the context of function types Void is also non-ambigious.

As @mkustermann suggested in #54739 (comment), we should consider making the type argument for @Native external functions omittable if it is pinned down by the Dart type:

@Native(isLeaf: true)
external void myFunc(Pointer<Int8>);

The only valid signature for this would be:

@Native<Void Function(Pointer<Int8>)>(isLeaf: true)
external void myFunc(Pointer<Int8>);

Which does not add any extra info.

And in contrast to the DynamicLibrary API, we also won't have a Pointer<NativeFunction<Void Function(Pointer<Int8>)>> type in our program that we would type check against.

[halildurmus]

I created a CL that implements this https://dart-review.googlesource.com/c/sdk/+/400840