feat: add support for external avatar hosting service (#614)

PluralKit.Bot/Services/AvatarHostingService.cs

@@ -0,0 +1,65 @@
+using System.Net;
+using System.Net.Http.Json;
+
+namespace PluralKit.Bot;
+
+public class AvatarHostingService
+{
+    private readonly BotConfig _config;
+    private readonly HttpClient _client;
+
+    public AvatarHostingService(BotConfig config, HttpClient client)
+    {
+        _config = config;
+        _client = client;
+    }
+
+    public async Task<ParsedImage> TryRehostImage(ParsedImage input, RehostedImageType type, ulong userId)
+    {
+        var uploaded = await TryUploadAvatar(input.Url, type, userId);
+        if (uploaded != null)
+        {
+            // todo: make new image type called Cdn?
+            return new ParsedImage { Url = uploaded, Source = AvatarSource.Url };
+        }
+
+        return input;
+    }
+
+    public async Task<string?> TryUploadAvatar(string? avatarUrl, RehostedImageType type, ulong userId)
+    {
+        if (!AvatarUtils.IsDiscordCdnUrl(avatarUrl))
+            return null;
+
+        if (_config.AvatarServiceUrl == null)
+            return null;
+
+        var kind = type switch
+        {
+            RehostedImageType.Avatar => "avatar",
+            RehostedImageType.Banner => "banner",
+            _ => throw new ArgumentOutOfRangeException(nameof(type), type, null)
+        };
+
+        var response = await _client.PostAsJsonAsync(_config.AvatarServiceUrl + "/pull",
+            new { url = avatarUrl, kind, uploaded_by = userId });
+        if (response.StatusCode != HttpStatusCode.OK)
+        {
+            var error = await response.Content.ReadFromJsonAsync<ErrorResponse>();
+            throw new PKError($"Error uploading image to CDN: {error.Error}");
+        }
+
+        var success = await response.Content.ReadFromJsonAsync<SuccessResponse>();
+        return success.Url;
+    }
+
+    public record ErrorResponse(string Error);
+
+    public record SuccessResponse(string Url, bool New);
+
+    public enum RehostedImageType
+    {
+        Avatar,
+        Banner,
+    }
+}