Add context for GraphQL and REST clients

[mszostok]

Description

Changes proposed in this pull request:

• Add context.Context for GraphQL client. For all methods there is alternative, with pattern {method_name}WithContext
  • Add unit test for context.Canceled and context.DeadlineExceeded to prevent any regression.
• Add context.Context for REST client. I added only DoWithContext and RequestWithContext. Rest methods are just a syntax sugar, and I don't see value into adding an alternative method to all of them. However, it's easy to add, so if you want to, let me know and I will do it 👍
  • Add unit test for context.Canceled and context.DeadlineExceeded to prevent any regression.

Related issue(s)

Fixes #47

[samcoe]

@mszostok This looks great, thanks for including tests with the PR!