Add RegisterNatives overload for raw function pointers

[simonrozsival]

Add JniNativeMethod struct matching JNI's JNINativeMethod layout (byte* Name, byte* Signature, IntPtr FunctionPointer) and a new RegisterNatives(JniObjectReference, ReadOnlySpan<JniNativeMethod>) overload.

Calls the JNI RegisterNatives function pointer directly via the function table, bypassing delegate marshaling entirely. Zero allocations in the entire path.

Add end-to-end test with a dedicated Java class (RegisterNativesTestType) and an [UnmanagedCallersOnly] native callback that is registered via the new API, then called from Java to verify the result.

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[jonathanpeppers]

##[error]src\Java.Interop\Java.Interop\JniEnvironment.Types.cs(288,99): Error CS1061: 'JniEnvironmentInfo' does not contain a definition for 'Invoker' and no accessible extension method 'Invoker' accepting a first argument of type 'JniEnvironmentInfo' could be found (are you missing a using directive or an assembly reference?)
D:\a\1\s\src\Java.Interop\Java.Interop\JniEnvironment.Types.cs(288,99): error CS1061: 'JniEnvironmentInfo' does not contain a definition for 'Invoker' and no accessible extension method 'Invoker' accepting a first argument of type 'JniEnvironmentInfo' could be found (are you missing a using directive or an assembly reference?) [D:\a\1\s\src\Java.Interop\Java.Interop.csproj]
##[error]src\Java.Interop\Java.Interop\JniNativeMethodRegistration.cs(30,23): Error RS0016: Symbol 'implicit constructor for 'JniNativeMethod'' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md)
D:\a\1\s\src\Java.Interop\Java.Interop\JniNativeMethodRegistration.cs(30,23): error RS0016: Symbol 'implicit constructor for 'JniNativeMethod'' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md) [D:\a\1\s\src\Java.Interop\Java.Interop.csproj]
##[error]src\Java.Interop\Java.Interop\JniNativeMethodRegistration.cs(36,10): Error RS0016: Symbol 'JniNativeMethod' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md)
D:\a\1\s\src\Java.Interop\Java.Interop\JniNativeMethodRegistration.cs(36,10): error RS0016: Symbol 'JniNativeMethod' is not part of the declared public API (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md) [D:\a\1\s\src\Java.Interop\Java.Interop.csproj]
##[error]src\Java.Interop\PublicAPI.Unshipped.txt(16,1): Error RS0017: Symbol 'Java.Interop.JniNativeMethod.JniNativeMethod(byte* name, byte* signature, System.IntPtr functionPointer) -> void' is part of the declared API, but is either not public or could not be found (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md)
D:\a\1\s\src\Java.Interop\PublicAPI.Unshipped.txt(16,1): error RS0017: Symbol 'Java.Interop.JniNativeMethod.JniNativeMethod(byte* name, byte* signature, System.IntPtr functionPointer) -> void' is part of the declared API, but is either not public or could not be found (https://github.com/dotnet/roslyn-analyzers/blob/main/src/PublicApiAnalyzers/PublicApiAnalyzers.Help.md) [D:\a\1\s\src\Java.Interop\Java.Interop.csproj]