Add MethodExtension to ServiceHandlerInstance

[Quinn-With-Two-Ns]

Add MethodExtension to ServiceHandlerInstance. This allows users, like the Temporal SDK, to define custom attributes on service handlers.

This will allow us to define Nexus Service like so.

[NexusServiceHandler(typeof(IStringService))]
public class TransferServiceImpl
{
    [TemporalOperation]
    public Task<TemporalOperationResult<string>> DoSomething(
        TemporalNexusOperationStartContext ctx, ITemporalNexusClient client, string input) =>
        client.StartWorkflowAsync(
            (TransferWorkflow wf) => wf.RunAsync(input),
            new() { Id = $"wf-{Guid.NewGuid()}" });
}

See also temporalio/sdk-dotnet#764

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Move MethodExtensionResult out of this file

This project treats analyzer warnings as errors via Directory.Build.props, and SA1402 is not disabled globally; adding MethodExtensionResult as a second top-level type in IMethodExtension.cs will trigger the same StyleCop rule that the existing multi-type OperationStartResult.cs has to suppress locally. Move this class to its own file or add an explicit suppression, otherwise builds that run analyzers fail before tests execute.

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