Reapply "Add image download proxy support to OneBot"

This reverts commit 2bff8343cd.

packages/napcat-onebot/config/config.ts

@@ -82,6 +82,7 @@ export const OneBotConfigSchema = Type.Object({
   musicSignUrl: Type.String({ default: '' }),
   enableLocalFile2Url: Type.Boolean({ default: false }),
   parseMultMsg: Type.Boolean({ default: false }),
+  imageDownloadProxy: Type.String({ default: '' }),
 });
 
 export type OneBotConfig = Static<typeof OneBotConfigSchema>;