DRAFT: Add example for async runtime

[pchickey]

Proposed changes

• First, rename existing async example to tokio. Its still fine to use tokio with ngx, but since this example was written we added an async executor to ngx that uses the nginx event loop, rather than using tokio in another thread.
• Then, make a copy of the tokio example named async where we remove all use of tokio, and replace it with ngx::async.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• 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 main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork