refactor: modularize Beam into new projects and interfaces

- Introduced modularity by splitting Beam into new projects: Beam.Abstractions, Beam.Models, and Beam.Downloaders.
- Refactored existing classes into appropriate namespaces and projects.
- Replaced specific implementations with abstractions (e.g., SourceLinkBuilder to LinkBuilder, State to IState, etc.).
- Updated interfaces: added ITemplate, IArticleData, IDownloadReport, and others for improved extensibility.
- Removed deprecated classes like SourceLinkBuilder and StateChangerFactory.
- Enhanced link handling in downloaders by refactoring to use `string` over `SourceLink`.
- Consolidated shared logic under Beam.Abstractions.

Beam.Api/ApiCall.cs

@@ -0,0 +1,66 @@
+using Microsoft.Extensions.Logging;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+
+namespace Beam {
+    public class ApiCall(HttpClient client, string uri, HttpMethod method, KeyValuePair<string, string[]>[] headers, object? requestData, object? body, params HashSet<HttpStatusCode> successCodes) {
+        public HttpClient Client { get; } = client;
+        public object? RequestData { get; } = requestData;
+        public object? Body { get; }
+        public string Uri { get; } = uri;
+        public HttpMethod Method { get; } = method;
+        public KeyValuePair<string, string[]>[] Headers { get; private set; } = headers;
+        public HashSet<HttpStatusCode> SuccessCodes { get; } = successCodes;
+
+        private string? ContentType = "application/json";
+
+        public async Task<ApiResponse> GetResponse(ILogger<ApiResponse>? logger, (int @try, int max)? tries = null, CancellationToken ct = default) {
+            SanitizeHeaders();
+            
+            var request = new HttpRequestMessage(Method, Uri);
+            request.Content = body is null ? request.Content :
+                body is string stringBody ? new StringContent(stringBody) : JsonContent.Create(body);
+
+            if (request.Content is not null)
+                request.Content.Headers.ContentType = ContentType is null ? null : new System.Net.Http.Headers.MediaTypeHeaderValue(ContentType);
+
+            foreach (var header in Headers)
+                request.Headers.Add(header.Key, header.Value);
+            
+            logger?.LogInformation("Fetching '{}' with method '{}', content-type '{}', and headers '{}'", Uri, Method, ContentType, JsonSerializer.Serialize(request.Headers.ToDictionary()));
+            var response = await Client.SendAsync(request, ct);
+
+            if (tries is not null && tries?.@try < tries?.max && !SuccessCodes.Contains(response.StatusCode)) {
+                await Task.Delay((int)Math.Min(Math.Pow(2, tries.Value.@try), 60) * 1000, ct);
+                return await GetResponse(logger, (tries.Value.@try + 1, tries.Value.max), ct);
+            }
+
+            return await ApiResponse.CreateAsync(response, logger, RequestData, ct);
+        }
+
+        private void SanitizeHeaders() {
+            Dictionary<string, string[]> headers = [];
+            foreach(var kvp in Headers) {
+                if (kvp.Value.Length == 0)
+                    continue;
+
+                if (kvp.Key == "Content-Type") {
+                    ContentType = kvp.Value[0];
+                } else {
+                    headers[kvp.Key] = kvp.Value;
+                }
+            }
+
+            Headers = headers.ToArray();
+        }
+
+        public static async Task<ApiResponse> Get(HttpClient client, string url, ILoggerFactory factory)
+            => await new ApiCall(client, url, HttpMethod.Get, [], null, null).GetResponse(factory.CreateLogger<ApiResponse>());
+    }
+}