[Bug]: Prevent blocking on `start_session` and `end_session`

### Contact Details

_No response_

### 📦 Package Version

0.3.18

### 🎞️ Framework Version

_No response_

### 🔎 Describe the Bug

Currently, `start_session` and `end_session` are blocking on the main thread. This is because start_session fetches a JWT which can take some time. We can move this to `agentops.init` to speed things up.

Additionally, `end_session` runs a flush. We can probably make this more of a "fire and forget" style signal. That is, when the `SessionExporter` sees an end_session event, it doesn't need to trigger a flush. We can just reserve the flush for SIGINT and program exit.

### 🤝 Contribution

- [ ] Yes, I'd be happy to submit a pull request with these changes.
- [ ] I need some guidance on how to contribute.
- [X] I'd prefer the AgentOps team to handle this update.