Add service collection extensions

[antoniovalentini]

Description

I had some time to play around with the deepl-client (you did an awesome job by the way) and I thought it may be useful to add a set of ServiceCollection extensions to streamline Translator usage combined with the .NET built-in dependency injection pipeline (probably related to the #26 issue).

In this PR I wrote a small POC, and here's how to use it:

configuration

// create a new service collection or use the .NET built-in one
var services = new ServiceCollection();

// load configuration from JSON files or environment variables
var config = LoadConfiguration();

// use the new `AddDeepL`
services.AddDeepL(config, options =>
{
    // and override the options based on your preferences
    options.appInfo = new AppInfo
    {
        AppName = "test-app",
        AppVersion = "0.0.1",
    };
});

actual usage

public class MyClass
{
    private readonly ITranslator _translator;

    public Impl(ITranslator translator)
    {
        _translator = translator;
    }

    public async Task<string> Translate()
    {
        const string sourceText = "Hello world!";
        return await _translator.TranslateTextAsync(sourceText, LanguageCode.English, LanguageCode.Italian);
    }
}

[antoniovalentini]

It's being included in the development branch.