[Feature] No message time in schema.Message and schema.AgenticMessage

**Is your feature request related to a problem? Please describe.**

Yes, I have to persist intermediate messages generated by deep-agent's subagents to database, but the message time is missing.

**Describe the solution you'd like**

When creating a new schema.Message or schema.AgenticMessage by eino framework, the create time can be recorded by a message field.

**Describe alternatives you've considered**

I can find all new messages by using callbacks before and after calling LLMs, and record current time, but it's not that elegent and can cause performance problems.

**Additional context**

Please consider my request seriously.