Extracts wasm and v8 workload executors into their own separate binary plugins

[autodidaddict]

This closes #363 , which switches from using Go-native plugins to simple binary plugins.

[autodidaddict]

Still have a bunch to do here as well as a lot of cleanup. Just checking this into a draft PR so I don't lose it all accidentally :)

[autodidaddict]

Things that still need to be done:

• make sure the rootfs stuff works by adding the wasm and v8 binaries to /usr/local/bin
• make sure node config defaults agent plugin path to /usr/local/bin
• finish messaging host service wasm client
• add and finish http client host service wasm client
• add documentation Pr to docs.nats.io containing new docs on wasm type
• explore a way to make it so the wasm runner is a single process with multiple modules rather than creating 1 process per module
• look into doing the same for v8

If I can't find a good way to to avoid spawning an 18MB (release build binary for Rust runner) per function, then this entire refactor might not be worth it and we might want to look at things from a different perspective. Of course, if we're running inside firecracker then we're forced to do process-per-function anyway (which leads to another tangent about maybe dedicating certain nex nodes to functions and others to native, so the function nodes can run out of sandbox)