Create a main-package proxy definition

[danielayala94]

On #1186, I proposed to declare a proxy stub for the Push Notifications Long Running Process.

Per @jonwis's guidance:

Can this be factored out into "the main-package proxy definition" and be where other future "things that talk to the main package" should go to live? Note that types here have to be carefully versioned compared to their framework-package brethren. LRP version 3 needs to be contactable by FXP versions 1-3. Then we have a single place to put proxy/remote'd API definitions.

Link to discussion: https://github.com/microsoft/WindowsAppSDK/pull/1186#discussion_r699685082