DynVm Implementation: Dynamically loaded native modules

[Protryon]

This PR adds a variant of NullVm, DynVM, that can load in native modules as dynamically linked modules.

Haven't written any automated tests yet, wanted to check if any design changes were going to be needed first, since the tests are going to be a bit more complicated than NullVM.

We've been testing this internally in an Envoy integration.

[PiotrSikora]

@mpwarres @kyessenov could one of you look at it? Thanks!

[PiotrSikora]

This PR adds a variant of NullVm, DynVM, that can load in native modules as dynamically linked modules.

Nitpicking, but if we're still talking about the NullVM, then I'd rather use "dynamically-loaded NullVM plugins" or similar, instead of adding another name for the NullVM.

[Protryon]

@mpwarres @kyessenov Any updates on this?

[Protryon]

@kyessenov @PiotrSikora I've pushed up some tests for DynVM.