[Misc] Ensure out-of-tree quantization method recognize by cli args

[liuyanyi]

#11969 support the register of out-of-tree quantization method, but the cli args cant load the updated QUANTIZATION_METHODS

In this pr, The validation of quantization methods (QUANTIZATION_METHODS) is now performed after loading all plugins in the post_init method of EngineArgs.
This change ensures that any newly registered quantization methods are available during validation.

This pr also allow write a quantization plugin in a general_plugin

For example:

@register_quantization_config("awq_turbomind")
class AWQTurbomindConfig(QuantizationConfig):

# Entrypoint
def register():
    from vllm.model_executor.layers.linear import WEIGHT_LOADER_V2_SUPPORTED

    # Register the quantization method
    from .turbomind_awq import (  # noqa: F401
        AWQTurbomindConfig,
        AWQTurbomindLinearMethod,
    )

    # This is needed for correct weight loading
    WEIGHT_LOADER_V2_SUPPORTED.append("AWQTurbomindLinearMethod")

After install this plugin, i can use the openai server without any code modify.

vllm serve qwen/qwq-32b-awq -q awq_turbomind

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