allow a priority flag on plugins tag to ensure correct order

I registered a Reply-To Plugin (set reply to for every message) but I couldnt see the correct message in the symfony profiler because the logger plugin was registered previously.

This little helper sorts the tagged services by priorty. No BC break - without priority flags there will be the same order.
findSortedByPriorityTaggedServiceIds() is called inside a loop which would result creating a new closure every time.

I think its better to use an `@internal` public method. But then again this compiler pass is only called during the cache-warming.