Go: implement Embed in GPUStack driver (#15182)

### What problem does this PR solve?

The Go GPUStack driver returned a stub error for `Embed()` even though
GPUStack exposes OpenAI-compatible embeddings on the **v1-openai** route
(not `v1/embeddings`).

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

conf/models/gpustack.json

@@ -2,7 +2,8 @@
   "name": "GPUStack",
   "url_suffix": {
     "chat": "v1/chat/completions",
-    "models": "v1/models"
+    "models": "v1/models",
+    "embedding": "v1-openai/embeddings"
   },
   "class": "local"
 }