feat: Add external GET /sources API (#1725)

HDX-3318

# Summary

This PR adds a `GET /sources` endpoint to the external API. This is intended to be a way for users to retrieve the list of available sources, so that they can use a correct source ID when creating or updating dashboards through the external API. Previously, the user had no easy way to view source IDs.

Create/Update/Delete source endpoints may be added in subsequent iterations.

There will be a related PR in control-plane to add this to the OpenAPI.

<img width="2126" height="1345" alt="Screenshot 2026-02-11 at 10 37 54 AM" src="https://github.com/user-attachments/assets/bd5ba25f-75df-495a-a25f-95b3a6a5cae2" />

```
curl --request GET \
  --url http://localhost:8000/api/v2/sources \
  --header 'authorization: Bearer <API Key>'
```

Author: pulpdrew

.changeset/chilly-boxes-grab.md

@@ -0,0 +1,5 @@
+---
+"@hyperdx/api": patch
+---
+
+feat: Add external GET /sources API