Resolvesfirezone/product#607
Setting the env var `CONNLIB_MOCK` when building through either
`build-rust.sh` or `gradle` will activate the `mock` feature.
This brindgs connlib from its own separated repo to firezone's monorepo.
On top of bringing connlib we also add and unify the Dockerfile for all
rust binaries and add a docker-compose that can run a headless client, a
relay and a gateway which eventually will test the whole flow between a
client and a resource. For this to work we also incorporated some elixir
scripts to generate portal tokens for those components.