HTTP mode: implement exec_timeout via context/cancel

[alexellis]

Description

Enable exec_timeout for http mode

Motivation and Context

This needed to be added to the http mode to control the duration of http requests

How Has This Been Tested?

Locally with Go outside of a container with a go function that sleeps within the timeout and beyond it to see that the error is given as Gateway timed out.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.