Feat: add gpustack model provider (#4469)

### What problem does this PR solve?

Add GPUStack as a new model provider.
[GPUStack](https://github.com/gpustack/gpustack) is an open-source GPU
cluster manager for running LLMs. Currently, locally deployed models in
GPUStack cannot integrate well with RAGFlow. GPUStack provides both
OpenAI compatible APIs (Models / Chat Completions / Embeddings /
Speech2Text / TTS) and other APIs like Rerank. We would like to use
GPUStack as a model provider in ragflow.

[GPUStack Docs](https://docs.gpustack.ai/latest/quickstart/)

Related issue: https://github.com/infiniflow/ragflow/issues/4064.

### Type of change

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



### Testing Instructions
1. Install GPUStack and deploy the `llama-3.2-1b-instruct` llm, `bge-m3`
text embedding model, `bge-reranker-v2-m3` rerank model,
`faster-whisper-medium` Speech-to-Text model, `cosyvoice-300m-sft` in
GPUStack.
2. Add provider in ragflow settings.
3. Testing in ragflow.

rag/llm/sequence2txt_model.py

@@ -13,6 +13,7 @@
 #  See the License for the specific language governing permissions and
 #  limitations under the License.
 #
+import os
 import requests
 from openai.lib.azure import AzureOpenAI
 import io
@@ -191,3 +192,14 @@ class TencentCloudSeq2txt(Base):
             return "**ERROR**: " + str(e), 0
         except Exception as e:
             return "**ERROR**: " + str(e), 0
+
+
+class GPUStackSeq2txt(Base):
+    def __init__(self, key, model_name, base_url):
+        if not base_url:
+            raise ValueError("url cannot be None")
+        if base_url.split("/")[-1] != "v1-openai":
+            base_url = os.path.join(base_url, "v1-openai")
+        self.base_url = base_url
+        self.model_name = model_name
+        self.key = key