Add Agones Game Server Client SDK

[aallbrig]

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/AgonesSDK
• Add a link to the dependency NuGet package: https://www.nuget.org/packages/Google.Api.CommonProtos
• Add a link to the dependency NuGet package: https://www.nuget.org/packages/Grpc
• It must have non-preview versions: 1.41.0 (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
• The lowest version added must be the lowest .NETStandard2.0 version available
• 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

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.

[aallbrig]

Issue this will fix: #375

[aallbrig]

@bdovaz I'm looking forward to making this contribution to open source and would appreciate your guidance. Thanks for the opportunity! Please review this PR when you can.

[aallbrig]

Need to take a look at why the build failed.

[aallbrig]

Added in AgonesSDK dependencies (Google.Api.CommonProtos, Grpc) as well. Updated top comment to include nuget package URLs there too.

The build is ready to be re-ran.

[aallbrig]

Closing this PR; made post on associated "issue" to explain why.