Add setter to HealthCheckOptions.ResultStatusCodes

All other properties (`Predicate`, `ResponseWriter` and `AllowCachingResponses`) have a setter but `ResultStatusCodes` doesn't.

Without a setter, reusing the same status to http status code mapping is impossible and leads to duplicate code that looks like this:

```csharp
private static void ConfigureHealthChecks(IApplicationBuilder app, HealthCheckServiceOptions options)
{
    app.UseHealthChecks("/health", new HealthCheckOptions
    {
        ResultStatusCodes =
        {
            [HealthStatus.Healthy] = StatusCodes.Status200OK,
            [HealthStatus.Degraded] = StatusCodes.Status400BadRequest,
            [HealthStatus.Unhealthy] = StatusCodes.Status503ServiceUnavailable
        }
    });
    foreach (var name in options.Registrations.Select(e => e.Name))
    {
        app.UseHealthChecks($"/health/{name}", new HealthCheckOptions
        {
            Predicate = registration => registration.Name == name,
            ResultStatusCodes =
            {
                [HealthStatus.Healthy] = StatusCodes.Status200OK,
                [HealthStatus.Degraded] = StatusCodes.Status400BadRequest,
                [HealthStatus.Unhealthy] = StatusCodes.Status503ServiceUnavailable
            }
        });
    }
}
```

With a setter, this code could be rewritten in a *don't repeat yourself* (DRY) way:

```csharp
private static void ConfigureHealthChecks(IApplicationBuilder app, HealthCheckServiceOptions options)
{
    var resultStatusCodes = new Dictionary<HealthStatus, int>
    {
        [HealthStatus.Healthy] = StatusCodes.Status200OK,
        [HealthStatus.Degraded] = StatusCodes.Status400BadRequest,
        [HealthStatus.Unhealthy] = StatusCodes.Status503ServiceUnavailable
    };
    app.UseHealthChecks("/health", new HealthCheckOptions
    {
        ResultStatusCodes = resultStatusCodes
    });
    foreach (var name in options.Registrations.Select(e => e.Name))
    {
        app.UseHealthChecks($"/health/{name}", new HealthCheckOptions
        {
            Predicate = registration => registration.Name == name,
            ResultStatusCodes = resultStatusCodes
        });
    }
}
```

src/Middleware/HealthChecks/src/HealthCheckOptions.cs

@@ -28,7 +28,7 @@ namespace Microsoft.AspNetCore.Diagnostics.HealthChecks
         /// Gets a dictionary mapping the <see cref="HealthStatus"/> to an HTTP status code applied to the response.
         /// This property can be used to configure the status codes returned for each status.
         /// </summary>
-        public IDictionary<HealthStatus, int> ResultStatusCodes { get; } = new Dictionary<HealthStatus, int>()
+        public IDictionary<HealthStatus, int> ResultStatusCodes { get; set; } = new Dictionary<HealthStatus, int>()
         {
             { HealthStatus.Healthy, StatusCodes.Status200OK },
             { HealthStatus.Degraded, StatusCodes.Status200OK },