feat(builders): multipart form data output support (#11248)

* feat(builders): multipart form data output support

* refactor: proper key management

* chore: add missing remarks

* chore: requested changes

* chore: rename encodables file

* chore: requested changes

* chore: requested changes

* chore: nits

Co-authored-by: Almeida <github@almeidx.dev>

* chore: requested change

* chore: requested change

---------

Co-authored-by: Almeida <github@almeidx.dev>

packages/util/src/RawFile.ts

@@ -0,0 +1,34 @@
+import type { Buffer } from 'node:buffer';
+
+/**
+ * Represents a file to be added to a request with multipart/form-data encoding
+ */
+export interface RawFile {
+	/**
+	 * Content-Type of the file.
+	 * If not provided, it will be inferred from the file data when possible
+	 *
+	 * @example 'image/png'
+	 * @example 'application/pdf'
+	 */
+	contentType?: string;
+	/**
+	 * The actual data for the file
+	 */
+	data: Buffer | Uint8Array | boolean | number | string;
+	/**
+	 * An explicit key to use for the formdata field for this file.
+	 * When not provided, the index of the file in the files array is used in the form `files[${index}]`.
+	 * If you wish to alter the placeholder snowflake, you must provide this property in the same form (`files[${placeholder}]`)
+	 */
+	key?: string;
+	/**
+	 * The name of the file. This is the actual filename that will be used when uploading to Discord.
+	 * This is also the name you'll use to reference the file with attachment:// URLs.
+	 *
+	 * @example 'image.png'
+	 * @example 'document.pdf'
+	 * @example 'SPOILER_secret.jpeg'
+	 */
+	name: string;
+}