Add MemoryPack

[nick-dodonov]

The NuGet package needs to respect a few constraints in order to be listed in the curated list:

• Add a link to the NuGet package:
  • https://www.nuget.org/packages/MemoryPack
  • https://www.nuget.org/packages/MemoryPack.Core
  • https://www.nuget.org/packages/MemoryPack.Generator
  • https://www.nuget.org/packages/MemoryPack.Streaming
  • https://www.nuget.org/packages/MemoryPack.UnityShims
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
  • Provided only .NETStandard2.1 assemblies
• The lowest version added must be the lowest .NETStandard2.0 version available
  • Provided lower version that was surely tested in real Unity builds (Win/macOS/Android/iOS/WebGL)
• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
  • The package was tested in standalone players: Win/macOS/Android/iOS/WebGL builds
• All package dependencies with .NETStandard 2.0 target must be added to the PR (respecting the same rules above)
  • Note that if a future version of the package adds a new dependency, this dependency will have to be added manually as well

Note: The server will be updated only when a new version tag is pushed on the main branch, not necessarily after merging this pull-request.