Tracing raw HTTP requests and responses in the client SDK

Let's add a context-manager interface to trace the raw HTTP requests and responses (e.g., logging them in the console) like:

```python
from ai.backend.client.session import AsyncSession

async with AsyncSession() as api_session:
async with api_session.trace(
handler=... # custom callback
) as trace:
await api_session.ComputeSession.get_or_create(...)
...
for reqres_log in trace: # retrieve the log afterwards
print(reqres_log)
```

This will be useful for debugging and interoperability development in external systems.

JIRA Issue: BA-166