`Memory<T>` / `Span<T>` support

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

In order to implement akkadotnet/akka.net#3740 Hyperion will need to support overloads that allow the external caller to control the allocation of memory using System.Memory types and buffer pools.

Describe the solution you'd like

Needs an actual spec, but leaving the issue here for record-keeping.