Add global override for $__interval

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: When zooming out on a chart, the automatically selected `$__interval` (`Time range / max data points`) might be wrong, or not what the user want to see. To fix this, people often introduce a custom `interval` variable and use it explicitly in the charts. This has multiple issues: - This variable has to be used instead of the already-provided `$__interval`, creating extra manual work and risk of inconsistencies. - Some plugins provide some extra macros that are using this default interval such as `$__interval_s`, `$__timeinterval` etc. for Clickhouse). They can't be used with a normal variable. - When graphs from different data source are used, the format for one interval might different (e.g., a `where` clause for clickhouse, vs. `[30s]` for promql), which would require the introduction of different variables, which gets very confusing. **What would you like to be added**: Add a global `Interval` selector next to the timeframe selector:  Something like that:  Mixpanel has a similar selector:  Perhaps it will need a notion of "default" or unset. **Who is this feature for?** All users.