…system

Implement CustomOp plugin system

Added passthrough Quant class

Update custom/brainsmith with qonnx main

Remove decorator/entry-point system back to domain-based system, but leveraging Python namespaces instead of custom_op dictionaries. This gives greater extensibility for disparate custom ops like in Brainsmith.

Replace namespace-based custom op registration with centralized registry
using (domain, op_type) keys. Add thread-safe operations with RLock,
lazy discovery, and caching. Deprecate is_finn_op and hasCustomOp in
favor of is_custom_op. Simplify add_op_to_domain signature to derive
op_type from class name. Update all call sites across codebase.

…e_with_main