FE: limit variable values by the selected time range

[prashant-shahi]

Currently all of the variables values are shown, irrespective of time range.

We could either provide flag to limit response over time range, or we could introduce start_time and end_time built-in variables which could be used in the CH query.

/cc @ankitnayan @palashgdev @srikanthccv

[srikanthccv]

• Users can use start_time and end_time in queries https://github.com/SigNoz/signoz/pull/5714
• We will cap the number of values in dropdown https://github.com/SigNoz/signoz/issues/5761

[srikanthccv]

Queries now can use the time range variables. The dynamic variable will do it automatically.