Add a local workflow for validating extension telemetry

## Summary
Add a supported local-development workflow for validating extension telemetry before publishing an extension.

## Context
Extension authors currently have no convenient way to inspect the telemetry they expect to emit. Consider one or more host-supported options, such as:
- a flag to dump telemetry locally to a file;
- forwarding spans to a local OpenTelemetry collector; or
- another documented inspection mechanism.

The workflow should make it possible to verify event names, attributes, and acceptance/drop behavior without sending telemetry to production backends.

## Reference
Follow-up from [PR #9174 review comment](https://github.com/Azure/azure-dev/pull/9174#discussion_r3754611516).