feat(signal-generator): add simulated gaps

[caterryan]

Summary

• add configurable simulated outage gaps to signal generator output
• document default gap behavior, disable flag, and upgrade impact
• bump signal generator to 0.3.0 and update registry metadata

Validation

• python3 -m pytest influxdata/signal_generator/test_signal_generator.py
• manually validated on influxdb:3-enterprise 3.10.2 with --without-auth and --plugin-dir=/plugins
• observed gap logs showing generated points removed and timestamp holes matching configured gap windows
• verified gap_enabled=false control trigger kept continuous 10 Hz output

[Aliaksei-Kharlap]

I don't think there's any need to add files with plans to the repository, same point for GAP_IMPLEMENTATION_PLAN.md

[caterryan]

Whoops. Good catch