Create decorator for manual AI agents instrumentation.

[matejminar]

Create a decorator that can be put on tools, agents, handoffs etc. If the decorator is there a span will created wrapping the function call.

Manual tracing in seer was introduced with getsentry/seer#2943

Actual sentry tracing is done by https://github.com/getsentry/seer/blob/0495ff48377b87587bc4b95ec99366982f318725/src/seer/ai_agent_tracing.py#L118

Resulting traces can be found in https://telemetry-experience.sentry.io/insights/agents/agents/?project=4509593331236944&statsPeriod=30d

[linear]

PY-1787 Ship decorator