Check classic and OTel agents against serverless

# Description
Serverless requires to do authentication sending a base64 encoded API key, double check that it is working with the classic agent and with the OTEL_EXPORTER_OTLP_HEADERS environment variable in our OTel distributions.

# Issues creation

## Create Spec issue?
- [x] Spec issue

## Create APM Agent issues?
- [x] elastic-otel-java
- [x] elastic-otel-dotnet
- [x] elastic-otel-node
- [x] elastic-otel-python
- [x] elastic-otel-php
- [x] apm-agent-java
- [x] apm-agent-dotnet
- [x] apm-agent-nodejs
- [x] apm-agent-python
- [x] apm-agent-go
- [x] apm-agent-php
- [x] apm-agent-ruby

Start by reviewing the serverless authentication requirement and the listed classic and OTel agent repositories. Verify base64 API-key authentication for the classic agents and the OTEL_EXPORTER_OTLP_HEADERS path in each listed OTel distribution; completion requires checks across all selected repositories, but no files or tests are named.

Written by the indexing model from the issue text.