### What problem does this PR solve?
This is a follow-up of #7088 , adding a knowledge base type input to the
`Begin` component, and a knowledge base selector to the agent flow debug
input panel:

then you can select one or more knowledge bases when testing the agent:

Note: the lines changed in `agent/component/retrieval.py` after line 94
are modified by `ruff format` from the `pre-commit` hooks, no functional
change.
### 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):