Add avatar configuration to server commands

[sylvesterkaczmarek]

Closes #5435

Summary

• Adds --avatar_config to adk web and adk api_server.
• Accepts an inline JSON object or a JSON file path.
• Validates input as google.genai.types.AvatarConfig before server startup.
• Applies the parsed configuration to live-session RunConfig instances.
• Adds tests for inline JSON, files, invalid input, both commands and live-run propagation.

Validation

• CLI/FastAPI suites → 245 passed, 5 skipped, 2 xfailed
• CLI option/signature suite → 8 passed
• Pre-commit checks on all changed files → passed

A remote avatar endpoint was not used.