Added an otel module image variant.

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