examples: adds context modification example

[chris-ramon]

Overview

• examples: example of a recommended alternative solution to modify context within parent resolvers to be available within children resolvers.
• closes: Modifying request context in a resolver #345, Modifying Context during a resolve function to transfer data down a tree #156,

Test plan

• Unit test

[coveralls]

Coverage decreased (-0.3%) to 80.025% when pulling 7ced578 on modify-context into fc25dbd on master.

[coveralls]

Coverage decreased (-0.3%) to 80.035% when pulling 7ced578 on modify-context into fc25dbd on master.