Add "context" option for pebble exec and exec health checks

[benhoyt]

Per discussion with Gustavo at our spec meeting last night, we'd like to add an API parameter context to pebble exec (command line arg --context) and to exec-type health checks, which would be the name of the service whose context to run the command in. It would use all the relevant context from the service definition, specifically environment variables and user and group.

Once we add working-dir we'll include that in the context too.

Note that we'd also like a way to say "inherit the context / env vars from pebble itself". We could do that with --context=- or --context=pebble or even --context= (empty string). We didn't come to a final decision on that syntax though.

Spec OP034.

[benhoyt]

Done in #246, though we decided to spell it "service-context".