Add methods to return server buffers as Memory<byte> as well as Span<byte>

[garethpotter]

Would be useful to be able to expose these as Memory<byte> as well as Span<byte> so that a reference to them can be held.

I am currently using this approach in https://github.com/brill-power/sunspec.net, where my generated models contain a reference to the holding register buffer which I can then manipulate through getters and setters.

[Apollo3zehn]

Thanks for your PR, I agree that the API needs a redesign. I am quite busy with work and private life but I am aware that I need to invest a week or so into this project to solve all the issues and review the merge requests that have accumulated so far. It will take a few weeks until I have more time and I hope that I then will finally be able to prepare v6 will all the proposed improvements included.