Support Cost Calculation in V0.2

[yiranwu0]

### Tasks
- [ ] #769
- [ ] https://github.com/microsoft/autogen/pull/812
- [ ] https://github.com/microsoft/autogen/pull/1269
- [ ] cost & usage summary in groupchat
- [ ] supporting cost calculation in assistant API
- [ ] Record the cost of each message. (In agents)
- [ ] Add time stamp to creation of each messages. (In agents)

Related:support logging #442, #412

[sonichi]

I like the plan. Thanks.