[Csharp SDK] Conformance test implementation

[markmandel]

Is your feature request related to a problem? Please describe.

There is unit tests for csharp, but not conformance tests. 😞

Describe the solution you'd like

Have a conformance test!

Will need to:

• Write an sdktest.sh in https://github.com/googleforgames/agones/tree/main/build/build-sdk-images/csharp that calls some C# code that calls the SDK in a way that conforms to how we expect requests to be sent (example below is the best reference).
• Add to sdk.mk a run-sdk-conformance-test-csharp target, and add it to run-sdk-conformance-tests target.

Examples:

• https://github.com/googleforgames/agones/blob/main/build/build-sdk-images/go/sdktest.sh
• https://github.com/googleforgames/agones/blob/main/test/sdk/go/sdk-client-test.go (which is called by above)

Describe alternatives you've considered

N/A - we should really have this.

Additional context

N/A