Help Wanted: Expand Embassy USB Examples

[jamesmunns]

As noted by @HaoboGu in #76 (comment), the port of code from postcard-rpc is probably not complete if you want to have other USB endpoints.

It would be great to:

• Extend/fix anything in the embassy-usb interface impl to allow for other endpoints
• Have a demo that does something else, like maybe run an rmk keyboard with a side Ergot channel for configuration/logs

[felipebalbi]

Just making sure folks have the correct link for embassy-use interface impl 👍