Plugin support

[AvivRubys]

IMO it makes a lot of sense for agogos to have plugin support, probably mostly for custom directives.
To properly support plugins, we should come up with a way to support all types of custom directives - both runtime directives and compile time directives (#9)
The trouble here is, that runtime directives run on the gateway which is written in Go, and compile-time directives probably should run on the registry after we stitch the schemas together, which runs on NodeJS.

WASM could work for both. Im slightly worried of running WASM on the hot path, but we can look into its performance implications.
Another options could be rewriting the registry in Go, which could also help with other things like better registry validation of schemas and less polyglotting needed to work on Agogos.