Add --feature-flag KEY=VALUE CLI argument that allows setting arbitrary
server feature flags at startup. Values are auto-converted to appropriate
Python types (bool, int, float, string). CLI flags are merged into
SERVER_FEATURE_FLAGS but cannot overwrite core flags.
Add --list-feature-flags which prints the registry of known CLI-settable
feature flags as JSON and exits, enabling launchers to discover valid
flags for a specific ComfyUI version.
Part of Comfy-Org/ComfyUI-Desktop-2.0-Beta#415
Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019d9386-54d3-74d9-a661-97e0a8d37b6b
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