Add context object to update status structs.

[nishkrishnan]

Idea here is to eventually have Context everywhere which allows us to use the k/v store that's part of it to pass middleware metadata around.

Basically I don't want us to necessarily have to pass our logger around everywhere just to have the same request scope. We should be able to pass in things like repo name, pull num, request id, through the context and initialize a new logger wherever we need.

Motivation for adding this came from needing the logger with request id in the gh client for logging status updates.