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/zh.ts

@@ -1210,6 +1210,8 @@ General：实体和关系提取提示来自 GitHub - microsoft/graphrag：基于
       promptMessage: '提示词是必填项',
       promptTip:
         '系统提示为大模型提供任务描述、规定回复方式，以及设置其他各种要求。系统提示通常与 key （变量）合用，通过变量设置大模型的输入数据。你可以通过斜杠或者 (x) 按钮显示可用的 key。',
+      knowledgeBasesTip: '选择关联的知识库，或者在下方选择包含知识库ID的变量。',
+      knowledgeBaseVars: '知识库变量',
     },
     footer: {
       profile: 'All rights reserved @ React',