Add ability to view and download spans for a session

### Description

AgentCore Starter Toolkit had a feature to view spans and traces for a session. It also downloaded spans to a JSON file while running on-demand evaluation. AgentCore CLI restricts these features to viewing traces across sessions and downloading a single trace.

Trace IDs are auto-generated and it's difficult to correlate them to Runtime session IDs. It would be simpler if I can list/view all the traces for a session and download all traces for a session.

### Acceptance Criteria

- [ ] Add a session ID filter option to `traces list` command
- [ ] Add a session ID option to `traces get` command that allows downloading all spans for a session

### Additional Context

_No response_