Support multiple proto path contexts, similar to kubectl context.

[marshallbrekka]

While our use-case largely revolves around a mego-repo of protos, it would be nice to support setting up multiple proto contexts (separate import and service paths) that could easily be switched between.

Maybe something like

contexts:
  name: project-a
  importPaths:
    - /my/dependencies
  servicePaths:
    - /my/services

currentContext: project-a

which could be switched between like so

$ gurl set-context project-a

[catherinetcai]

This, or setting context via url:

gurl -u k8://context/service:port/rpc/method

[marshallbrekka]

@catherinetcai i think we're talking about two different kinds of contexts. One is which protos/services you want to be using, the other is which kubernetes cluster you want to be connecting to.