CI check detecting old extrinsic weights

# Background

Currently, it may unfortunately be very easy to do a change to a codepath for one or more extrinsic, postpone updating benchmarks, and then at a much later time forget to update benchmarks for one or more extrinsics. Having some way to detect this would be very beneficial.

# Proposal

Integrate a script into the CI which reviews the diff between target and source branch, identifies all extrinsics affected by a change (not trivial), and then checks if weight is also updated. Perhaps there is some other easier way, but seems like some solution is needed here.