Define the future for VSecM

[v0lkan]

Both SPIKE and VSecM targets similar things, and SPIKE will be containerized soon.

It seems fitting to use SPIKE as an upstream pluggable first-class-citizen provider to VSecM, while also keeping things backwards-compatible.

^ that's one opiton.

Another option is to use SPIKE as the backing store while keeping things almost (but not exactly) compatible.

In either case, we need to document our plan.