Add context (context.Context) for global variable get/set

[YangKeao]

Enhancement

The hook of set/get global variable doesn't have a context.Context argument. However, it theoretically could have (from the set statement).#38274 will add a set/get hook which reads from the pd, and it requires the hook has an argument context.Context.

Besides the direct set statement, some codes of tidb use the same functions to set/get the variables directly, and they don't have a context.Context in near stacks. So I'll only add context for the set, show, select statement execution, but not for every usage.