Add golang-otel template

[welteki]

Description

Add a new template named golang-otel that has built-in support for OpenTelemetry traces.
The template is based on the golang-middleware template.

• Supports exporter configuration through environment variables. (Similar to otel auto-instrumentation agent for other languages.)
• Supports otlp and console exporters.
• Instruments the function handler automatically using the otelhttp package.
• Allows users to easily add custom traces.

How Has This Been Tested?

Verified the template in various examples.

How are existing users impacted? What migration steps/scripts do we need?

This change adds a new template and does not directly impact existing users.

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• read the CONTRIBUTION guide
• signed-off my commits with git commit -s
• added unit tests