simulate: audio subcommand with impairment flags

[u9g]

Turns --audio into an audio subcommand, which carries three new booleans sent on the create-simulation message:

lk agent simulate audio \
  --scenarios scenarios.yaml \
  --background-noise \
  --low-quality-microphone \
  --packet-loss

simulate's own flags did not need duplicating on the subcommand: urfave/cli v3 flags are persistent unless Local is set, so --scenarios, --agent-name, -n and friends parse on either side of audio. Since v3's help only renders the root's persistent flags under GLOBAL OPTIONS, the subcommand carries a one-line description pointing back at them.

Verified against a local fake twirp endpoint: lk agent simulate audio --background-noise --low-quality-microphone --packet-loss sends

mode: SIMULATION_MODE_AUDIO
background_noise: true
low_quality_microphone: true
packet_loss: true

while plain lk agent simulate still sends SIMULATION_MODE_TEXT with no impairments, and the impairment flags are rejected on the parent command.

Draft, blocked on livekit/protocol#1723: the fields land on SimulationRun.Create.Request there, so CI cannot compile until that publishes and the dependency is bumped here.

Breaking: --audio shipped in v2.18.2 and is removed by this change, replaced by the subcommand. Say the word if it should stay as a hidden alias.