New injector

[prdoyle]

Implements ES-9076.

There are two notable commits here:

• 1de4b74bb181ba223f91fd4c87e8666f861a3e99 adds the ability for createComponents to return classes, which the new injector will inject. This feature takes the place of the build-time scan, which I've left out of this PR, but we anticipate re-adding later, so this feature is effectively temporary (hopefully!)
• 2e4c39343fdd6f35a925df9323e3d8ffbf5aad7b uses the new injector for one of the simplest plugins, Downsample. It demonstrates the kind of changes we can do now with the new injector.