refactor: allow modification of the query mode

[zwang28]

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What's changed and what's your intention?

This PR permits to set the runtime parameter query_mode gloablly. Sometimes it's helpful to easily alter this parameter globally for queries sent from a client process, which otherwise need code change and rebuild for client process.

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[BugenZhao]

May I know why it was previously set to NO_ALTER_SYS?

[zwang28]

May I know why it was previously set to NO_ALTER_SYS?

We've discussed offline and agreed NO_ALTER_SYS is not necessary.

@yuhao-su

[lmatz]

It seems our documentation and the runtime parameter system table do not differentiate if a parameter can be set globally or not at the moment. I thought all can be alter system set