Correctly bundle Sailthru Models DLL in Nuget package

[jethrocarr]

With the recent move to use .Net Standard 2.0, we broke the bundling of Sailthru.Models library into the Nuget package.

Correctly including dependencies seems to be a repeat pain point for Nuget as per dotnet/sdk#6688, but the approach added here seem to work and can see the Model now being included in the compiled package

% unzip -l Sailthru.Client.1.2.1.nupkg
Archive:  Sailthru.Client.1.2.1.nupkg
  Length      Date    Time    Name
---------  ---------- -----   ----
      505  12-20-2022 13:59   _rels/.rels
      812  12-20-2022 13:59   Sailthru.Client.nuspec
    30208  12-20-2022 00:21   lib/netstandard2.0/Sailthru.Models.dll <-- was previously missing
    23040  12-20-2022 00:59   lib/netstandard2.0/Sailthru.dll
      459  12-20-2022 13:59   [Content_Types].xml
      644  12-20-2022 13:59   package/services/metadata/core-properties/fae2406158514c48b501b0980e09e4f2.psmdcp
---------                     -------
    55668                     6 files

Additionally Sailthru.Models is no longer a dependency inside Sailthru.Client.nuspec file so once uploaded Nuget should stop trying to reference it as a dependency to download.

Also fixed the license warning for good measure.