Add async endpoint support

[viveksyngh]

This commit adds async endpoint support to cron connector which can be enabled using async_invocation environment variable

Description

Motivation and Context

• [ ] I have raised an issue to propose this change (required)
• [x] My issue has received approval from the maintainers or lead with the design/approved label

11

How Has This Been Tested?

Tested on my local system

k logs cron-connector-79d479b777-bvh7w -n openfaas -f
2021/03/20 17:20:54 Version: 43da735bc8f99800a52718a3e5ac31c4884349b7   Commit: dev
2021/03/20 17:20:54 Gateway URL:  http://gateway.openfaas:8080
2021/03/20 17:20:54 Async Invocation:  true
2021/03/20 17:21:04 added function nodeinfo in openfaas-fn
2021/03/20 17:25:00 Executed function: nodeinfo (ns=openfaas-fn)

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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.
• [ ] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[derek[bot]]

Thank you for your contribution. I've just checked and your commit doesn't appear to be signed-off. That's something we need before your Pull Request can be merged. Please see our contributing guide. Tip: if you only have one commit so far then run: git commit --amend --signoff and then git push --force.

[alexellis]

LGTM, next could you:

• Sign-off the commit(s)
• Fill out How Has This Been Tested?

And we should be able to finally ship this 🙂

[viveksyngh]

@alexellis Added missing parts :)