fix(auth): propagate headers from source request

[Umaaz]

What this PR does:

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[github-actions]

Levitate is-compatible report:

🔍 Resolving @grafana/data@latest...
🔍 Resolving @grafana/ui@latest...
🔍 Resolving @grafana/runtime@latest...
🔍 Resolving @grafana/schema@latest...
🔍 Resolving @grafana/e2e-selectors@latest...
🔍 Resolving @grafana/experimental@latest...

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/data version 10.3.3 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/ui version 10.3.3 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/runtime version 10.3.3 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/schema version 10.3.3 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/e2e-selectors version 10.3.3 locally

🔬 Checking compatibility between ./src/module.ts and @grafana/[email protected]...
✔ Found @grafana/experimental version 1.7.10 locally

✔️ ./src/module.ts appears to be compatible with @grafana/data,@grafana/ui,@grafana/runtime,@grafana/schema,@grafana/e2e-selectors,@grafana/experimental