Feature/del 2757 languages in sdk

[Sevitas]

Motivation

Introducing new listing endpoint for Languages in Delivery API and adding support for it in SDK

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docs have been updated (if applicable)
• Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

Check that method returns only active languages for project

[codecov]

Codecov Report

Merging #256 (5d8df7d) into master (3f60637) will decrease coverage by 0.04%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
- Coverage   80.67%   80.63%   -0.05%     
==========================================
  Files         108      111       +3     
  Lines        1573     1611      +38     
  Branches      452      465      +13     
==========================================
+ Hits         1269     1299      +30     
  Misses         88       88              
- Partials      216      224       +8     

Impacted Files	Coverage Δ
Kentico.Kontent.Delivery.Caching/CacheHelpers.cs	41.79% <20.00%> (-1.76%)	⬇️
...ico.Kontent.Delivery.Rx/DeliveryObservableProxy.cs	81.57% <50.00%> (-1.76%)	⬇️
Kentico.Kontent.Delivery/DeliveryClient.cs	66.88% <71.42%> (+0.21%)	⬆️
...co.Kontent.Delivery.Caching/DeliveryClientCache.cs	86.20% <85.71%> (-0.07%)	⬇️
....Delivery.Abstractions/DeliveryClientExtensions.cs	100.00% <100.00%> (ø)
...ivery/Languages/DeliveryLanguageListingResponse.cs	100.00% <100.00%> (ø)
Kentico.Kontent.Delivery/Languages/Language.cs	100.00% <100.00%> (ø)
...ent.Delivery/Languages/LanguageSystemAttributes.cs	100.00% <100.00%> (ø)
...ontent.Delivery/Urls/DeliveryEndpointUrlBuilder.cs	97.29% <100.00%> (+0.07%)	⬆️
... and 2 more

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[petrsvihlik]

this abstraction should know nothing about the objects that implement it. therefore even this comment seems to be irrelevant.

[petrsvihlik]

Could we be a bit more specific? Something like "returns all active languages assigned to a given project and matching the optional filtering parameters."...

[petrsvihlik]

again, a better comment here would be appreciated.

[petrsvihlik]

docs: https://kentico.atlassian.net/browse/CTC-1199

[petrsvihlik]

tested with:

using Kentico.Kontent.Delivery.Abstractions;
using Kentico.Kontent.Delivery;
using System;
using Kentico.Kontent.Delivery.Builders.DeliveryClient;
using System.Threading.Tasks;
using Kentico.Kontent.Delivery.Urls.QueryParameters.Filters;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Program
    {
        static async Task Main(string[] args)
        {
            IDeliveryClient deliveryClient = DeliveryClientBuilder
                .WithOptions(o => o.WithProjectId("964d51ec-d07d-00cf-5aa5-7ee52ef970dc")
                .UseProductionApi()
                .WithCustomEndpoint("https://secret-endpoint.global.ssl.fastly.net/{0}")
                .Build()).Build();

            var lng = await deliveryClient.GetLanguagesAsync(new List<IQueryParameter>() { new EqualsFilter("system.name", "Czech")});

            foreach (var l in lng.Languages)
            {
                Console.WriteLine(l.System.Name);
            }
        }
    }
}