[CommunityToolkit.Maui.Markup.Sample] Replace Refit + Polly with Microsoft.Extensions.Http.Polly and Refit.HttpClientFactory

[brminnick]

To demonstrate best practices for .NET MAUI apps that utilize HttpClient, this PR replaces the following NuGet Packages:

• Refit -> Refit.HttpClientFactory
• Polly -> Microsoft.Extensions.Http.Polly

Because .NET MAUI supports dependency injection, we can leverage Refit.HttpClientFactory to easily implement REST API requests and Microsoft.Extensions.Http.Polly to add automatic error handling policies. In the updated code below, when HttpClient fails it will now automatically retry to connect to the URL 3 times using an exponential backoff of 2ⁿ seconds. This is ideal for flaky mobile internet connections where a user's device may switch cellular towers or lose a Wifi connection.

builder.Services.AddRefitClient<IHackerNewsApi>()
	.ConfigureHttpClient(client => client.BaseAddress = new Uri("https://hacker-news.firebaseio.com/v0"))
	.AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(3, sleepDurationProvider));

static TimeSpan sleepDurationProvider(int attemptNumber) => TimeSpan.FromSeconds(Math.Pow(2, attemptNumber));