Add Akka.Streams and Reactive.Streams

[bdovaz]

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• Add a link to the NuGet package: https://www.nuget.org/packages/XXX
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• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
  • if the package is not compatible with standalone player, please add a comment to a Known issues section to the top level readme.md
• 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

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