Feat: Support passing knowledge base id as variable in retrieval component (#7088)

### What problem does this PR solve?

Fix #6600

Hello, I have the same business requirement as #6600. My use case is: 

We have many departments (> 20 now and increasing), and each department
has its own knowledge base. Because the agent workflow is the same, so I
want to change the knowledge base on the fly, instead of creating agents
for every department.

It now looks like this:


![屏幕截图_20250416_212622](https://github.com/user-attachments/assets/5cb3dade-d4fb-4591-ade3-4b9c54387911)

Knowledge bases can be selected from the dropdown, and passed through
the variables in the table. All selected knowledge bases are used for
retrieval.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

agent/component/retrieval.py

@@ -41,6 +41,7 @@ class RetrievalParam(ComponentParamBase):
         self.top_n = 8
         self.top_k = 1024
         self.kb_ids = []
+        self.kb_vars = []
         self.rerank_id = ""
         self.empty_response = ""
         self.tavily_api_key = ""
@@ -58,7 +59,22 @@ class Retrieval(ComponentBase, ABC):
     def _run(self, history, **kwargs):
         query = self.get_input()
         query = str(query["content"][0]) if "content" in query else ""
-        kbs = KnowledgebaseService.get_by_ids(self._param.kb_ids)
+
+        kb_ids: list[str] = self._param.kb_ids or []
+
+        kb_vars = self._fetch_outputs_from(self._param.kb_vars)
+
+        if len(kb_vars) > 0:
+            for kb_var in kb_vars:
+                if len(kb_var) == 1:
+                    kb_ids.append(str(kb_var["content"][0]))
+                else:
+                    for v in kb_var.to_dict("records"):
+                        kb_ids.append(v["content"])
+
+        filtered_kb_ids: list[str] = [kb_id for kb_id in kb_ids if kb_id]
+
+        kbs = KnowledgebaseService.get_by_ids(filtered_kb_ids)
         if not kbs:
             return Retrieval.be_output("")
 
@@ -75,7 +91,7 @@ class Retrieval(ComponentBase, ABC):
             rerank_mdl = LLMBundle(kbs[0].tenant_id, LLMType.RERANK, self._param.rerank_id)
 
         if kbs:
-            kbinfos = settings.retrievaler.retrieval(query, embd_mdl, kbs[0].tenant_id, self._param.kb_ids,
+            kbinfos = settings.retrievaler.retrieval(query, embd_mdl, kbs[0].tenant_id, filtered_kb_ids,
                                         1, self._param.top_n,
                                         self._param.similarity_threshold, 1 - self._param.keywords_similarity_weight,
                                         aggs=False, rerank_mdl=rerank_mdl,
@@ -86,7 +102,7 @@ class Retrieval(ComponentBase, ABC):
         if self._param.use_kg and kbs:
             ck = settings.kg_retrievaler.retrieval(query,
                                                    [kbs[0].tenant_id],
-                                                   self._param.kb_ids,
+                                                   filtered_kb_ids,
                                                    embd_mdl,
                                                    LLMBundle(kbs[0].tenant_id, LLMType.CHAT))
             if ck["content_with_weight"]: