Some internal http client call rely on methods that do not support context

[gdbranco]

What

Current implementation for oauth2 repo, specifically jwt, jira and google/downscope packages rely on .PostForm function which does not allow to specify a context.

Problem

Forwarding the context provides consumers more control over timeout/cancel, allows to leverage context values in log message wrappers etc

Proposal

The context is available within the function and preferably should be used to give more control to the consumer

[gdbranco]

I went ahead and setup a PR #770 covering the work for this refactor

Edit: I'm looking through the contributing guide to follow the proper flow