Akka.Util: improve Result<T>

[Aaronontheweb]

Changes

Designed to support our work on #7518

Makes the following changes to Result<T>:

• Enables nullability
• Converts fields to properties (should have been that way from the beginning IMHO) - this might be a binary compatibility issue but I doubt it though as this is mostly used internally by Akka.Streams and other components.
• Redesigns the struct to be a readonly record struct so it an automatically benefit from equality by value and other compiler optimizations

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Changes in public API reviewed, if any.

[Aaronontheweb]

API change - make fields into nullable properties.

[Arkatufus]

LGTM

[Aaronontheweb]

Reverted back to using fields here instead of properties, out of an abundance of caution WRT binary compatibility