App extension API only doesn't need to be required

[noahsmartin]

Problem Statement

Right now we set APPLICATION_EXTENSION_API_ONLY=YES because of the side effect that it makes internal swift public There's no guarantee this will stay the case, and it's definitely not what this setting is meant for.

Solution Brainstorm

Instead we could make the swift types that should be exposed to objc public which is the documented way to do it, and add @_spi(Private) to keep this code not visible in public swift code. We've already done some of this here: #5307 If we keep updating this swift code to use @_spi we'll be able to remove APPLICATION_EXTENSION_API_ONLY=YES from our config files.

Are you willing to submit a PR?

No response