Support NGINX Variables

[mpstefan]

### Tasks
- [ ] Design for Allowed NGINX Variables in Request and Response Headers
- [ ] #2040 
- [ ] Allow set of NGINX Variables for ResponseHeaderModifier
- [ ] Allow set of NGINX Variables for OpenTelemetry Attribute

[sjberman]

@mpstefan Since this also includes Otel attributes, I would simplify the title of this epic (since that's not related to header filters).

I also suspect this could be something that gets bumped up in priority. Being able to include variables offers a lot of value in gathering or adding data to requests that could be very useful for debugging issues or just for general information.

[nginx-bot]

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