Consider replacing all Marshal.PtrToStructure with manual marshalling

[Fabi]

Recently I worked on some apps with sharppcap and tested it with the reflection free mode in native aot. Currently it's incompatible due to the usage of Marshal.PtrToStructure. It should be considered to change all those usages to custom marshalling to have those things working at least in .NET 7 in reflection free mode.

Samples from a runtime change: https://github.com/dotnet/runtime/pull/70866/files