Added an otel module image variant.

[thresheek]

Proposed changes

The variant provides an image built with a recently opensourced opentelemetry module developed by F5/NGINX [1].

I've decided to make a separate variant based on the main image instead of extending it because the module build-depends on a fairly large chunk of C++ code from multiple projects, which takes around 10 minutes to compile and link on an 8-core amd64 machine. This is why it's currently limited to amd64 and arm64v8, which nginx.org provides builds for. Users can build them on less popular architectures as the instructions are still provided in the dockerfiles.

Also, it's currently only available for the "mainline" branch, with "stable" to follow in the future.

[1] https://nginx.org/en/docs/ngx_otel_module.html
[2] https://nginx.org/en/linux_packages.html#dynmodules

Checklist

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

• I have read the CONTRIBUTING document
• I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation

[thresheek]

cc @tianon @yosifkit

I plan to merge this and submit to the official library - can you let me know what are your feelings about it?

Thanks!