csharp SDK: exposes the Alpha SDK via the main interface

[rcreasey]

What type of PR is this?

/kind bug

What this PR does / Why we need it:
Exposes the alpha sdk object for those who implement their code against the main IAgonesSDK interface in the csharp SDK.

This is for classes like so:

public class AgonesStateService {
    private readonly IAgonesSDK m_agonesClient;
    public AgonesStateService(IAgonesSDK agonesClient) {
      m_agonesClient = agonesClient ?? new AgonesSDK();
    }
}

I want to be able to access the Alpha SDK methods like this:

private async Task<bool> PlayerConnectAsync(string playerId) {
    await m_agonesClient.Alpha().PlayerConnectAsync(playerId);
}

Without this change, I would have to implement code directly against the AgonesSDK.

Special notes for your reviewer:
(If there is another intended way this was to be exposed, let me know. I wish I would have suggested this in the original PR)

[markmandel]

Approved!

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, rcreasey

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• OWNERS [markmandel]

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[agones-bot]

Build Succeeded 👏

Build Id: bcaa7fe6-6f25-4690-81a8-015e13c4ec68

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.10.0-f8db718
• image: gcr.io/agones-images/agones-sdk:1.10.0-f8db718
• image: gcr.io/agones-images/agones-ping:1.10.0-f8db718
• Linux C++ SDK (build): agonessdk-1.10.0-f8db718-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.10.0-f8db718.zip

A preview of the website (the last 30 builds are retained):

• https://f8db718-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1896/head:pr_1896 && git checkout pr_1896
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.10.0-f8db718