add async rust example using tokio

[sahandevs]

Proposed changes

This PR adds a simple example how to integrate rust async (using tokio) with nginx event loop.

Checklist

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[dekobon]

Merged in: #63

[dekobon]

Thank you for your contribution!