[API Proposal]: ARM64-SVE: Masked versions of Across methods

**TODO: This issue needs filling out with other Across methods....**

```csharp
namespace System.Runtime.Intrinsics.Arm;

/// VectorT Summary
public abstract partial class Sve : AdvSimd /// Feature: FEAT_SVE  Category: maths
{
  /// T: float, double, long, ulong
  public static unsafe Vector<T> AddAcross(Vector<T> mask, Vector<T> value); // FADDV or UADDV // predicated

}
```