WP-NOW: Simplify multipart requests by dropping manually encoding

[fluiddot]

What?

Drop manually encoding multipart requests. This PR is a follow-up of https://github.com/WordPress/playground-tools/pull/203.

Why?

This PR follows this discussion regarding the necessity of manually encoding multipart requests. Additionally, it addresses https://github.com/WordPress/playground-tools/issues/259.

How?

• Remove Express fileUpload middleware to avoid modifying the request object.
• Remove encodeAsMultipart helper, since we don't need to process them differently (Playground CLI example).

Testing Instructions

1. Run the command: nvm use && npm install.
2. Run the command nx preview wp-now start.
3. Open the WP-admin page (e.g. http://localhost:8881/wp-admin).
4. Navigate to the Media page.
5. Upload a media file.
6. Observe the media has been uploaded correctly to the Media Library.