Plugin::build should accept self not &self

## What problem does this solve or what need does it fill?

Currently to drill an allocated object into plugins you either need to use a resource or `Arc>`. It would nice for the default, most common case to allow consuming plugin content on build..

## What solution would you like?

The `Plugin` trait should only have one method: `build(self, &mut App)`. The existing `Plugin` trait should be renamed to `FullPlugin`, and its methods should be `&mut` not `&`.

## What alternative(s) have you considered?

Continue using resources/`Arc>`.

## Additional context

IIRC the current `&self` restriction is from the old plugin design. Don't see any reason or docs saying the plugin methods can't be mutable.