### What problem does this PR solve?
Use task save function for add_message api, and added http API document.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] Documentation Update
### What problem does this PR solve?
Adapt to ',' joined arg list in get method url.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Manage message and use in agent.
Issue #4213
### Type of change
- [x] New Feature (non-breaking change which adds functionality)