Implement allow_feature_tier Global Switch to Disable Experimental Features

Why it matters

This feature provides a centralized control mechanism to easily disable all experimental and beta features at once, improving stability and control over which features are active in production environments.

How to use it

Users can control the availability of experimental and beta features by setting the allow_feature_tier parameter in the server configuration or at the session level. Setting it to disable will turn off all these features globally.