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):

web/src/locales/en.ts

@@ -1255,6 +1255,9 @@ This delimiter is used to split the input text into several text pieces echo of
       promptTip:
         'Use the system prompt to describe the task for the LLM, specify how it should respond, and outline other miscellaneous requirements. The system prompt is often used in conjunction with keys (variables), which serve as various data inputs for the LLM. Use a forward slash `/` or the (x) button to show the keys to use.',
       promptMessage: 'Prompt is required',
+      knowledgeBasesTip:
+        'Select the knowledge bases to associate with this chat assistant, or choose variables containing knowledge base IDs below.',
+      knowledgeBaseVars: 'Knowledge base variables',
     },
   },
 };