Add context propagation middleware

/to @ritch @bajtos @piscisaureus 
> If we decide to enable context by default, I would like to change the loopback workspace api-server template so that the default values of related configuration properties will show in the `server/config.json` so that our users can opt out certain features easily.

:+1: for modifying loopback-workspace templates.

My understanding is that we need the context to implement features like propagating user credentials to downstream services, e.g. when making a SOAP or a REST call to backends. Unless we enable the context middleware by default, all modules depending on the context will have to implement a check to ensure that the context was created and the documentation should include instructions on how to enable the context middleware. It will make the initial experience unnecessarily complex IMO.