Added resolveImage to reduce code duplication (#1820)

* add client#resolveImage

* oops

* resolveFile fix

* async is the future

* async

* update doc example

* build fix

* doc fix

* fix docs

* thx hydar

src/client/ClientDataResolver.js

@@ -177,6 +177,20 @@ class ClientDataResolver {
     return data;
   }
 
+  /**
+   * Resolves a Base64Resolvable, a string, or a BufferResolvable to a Base 64 image.
+   * @param {BufferResolvable|Base64Resolvable} image The image to be resolved
+   * @returns {Promise<string>}
+   */
+  async resolveImage(image) {
+    if (!image) return null;
+    if (typeof image === 'string' && image.startsWith('data:')) {
+      return image;
+    }
+    const file = await this.resolveFile(image);
+    return this.resolveBase64(file);
+  }
+
   /**
    * Data that resolves to give a Base64 string, typically for image uploading. This can be:
    * * A Buffer