Unity IL2CPP and New CodeGenerator

[neuecc]

About #568, includes fix #559, #560, #562, #564

I've added new code generator MessagePack.Generator and MessagePack.MSBuild.Tasks.
MessagePack.UniversalCodeGenerator is deperecated.

MessagePack.Generator can be dotnet core global tools so require to add nuget push script to ci.
MessagePack.MSBuild.Tasks is similar, so also require to add nuget push script to ci.

New generator is port from #396.

[neuecc]

IL2CPP test can run on CI.

- run:
    name: Build Windows(IL2CPP)
    command: /opt/Unity/Editor/Unity -quit -batchmode -nographics -silent-crashes -logFile -projectPath . -executeMethod UnitTestBuilder.BuildUnitTest /headless /ScriptBackend IL2CPP  /BuildTarget StandaloneWindows64
    working_directory: MessagePack.UnityClient
# execute headless player on CI
- run: MessagePack.UnityClient/bin/UnitTest/StandaloneWindows64_IL2CPP/test.exe

[neuecc]

and Added StaticCompositeResolver which is similar as v1's CompositeResolver.
Because v2's CompositeResolver does not run on IL2CPP, simple is always best:)

[jsteinich]

For our use case this is a blocker, but I understand that it isn't trivial to implement.

[neuecc]

Yes, I should support it but it will be a later.
Old mpc has many issues so must move to this model in v2.

[AArnott]

I mostly reviewed the common MessagePack code. I didn't spend much time on the MPC-specific stuff. Looks good.

[neuecc]

thanks.
I'll add doc comment(maybe there comment will be poor:)) and will add some fix to create unitypackage.
after, do merge.

[neuecc]

commented and add some fixes, it works in my local.
I don't know CI fails.

[AArnott]

The CI is failing with several errors that were warnings in VS' error list including these two:

Error VSTHRD111: Add .ConfigureAwait(bool) to your await expression.
Error RS0016: Symbol 'QueueFormatter' is not part of the declared API.

The first one is because of #595 which ensures we don't accidentally forget .ConfigureAwait(false) that can result in deadlocks when GUI apps use MessagePack and synchronously block on the result.

The second one is because of #596 which requires that all public API changes to the MessagePack library now be explicitly declared in a PublicAPI.*.txt file, as explained in that PR.

Both of these warnings have code fix providers to make fixing all of them very easy: