Opentelemetry-js: Better plugin organization: take into account common vs node vs web plugins

[clehene]

The contrib has a separation already https://github.com/open-telemetry/opentelemetry-js-contrib/tree/master/plugins
However, I'm guessing core would also have some common parts (the "core" parts :) )
Also
grpc-js is only for node, but there's also a grpc plugin which is also for node. Not sure what the difference is.
It should be easy for someone that tries to create a plugin to understand what the structure of what needs to be implemented looks like.

[dyladan]

I think we should probably split this into web, node, and common directories. Not just for plugins but for the whole project.

As far as plugins go, they should go into contrib.