Add extra statistic to track the number of signature validation operations

[rijswijk]

This PR adds an extra statistic to the extended statistics to track the number of signature validation operations performed by the validator module. This is mostly useful for research purposes, as it makes it possible to track the workload required for cryptographic operations over time.

The statistic tracks the number of calls to verify_canonrrset(..) in val_sigcrypt.c. The implementation follows the same pattern as for the statistic that tracks the number of bogus RRsets. That is: there is a lock around operations that change or read the statistic (I assume this is because the validator module is not replicated in each thread if Unbound runs multi-threaded).

It would be much appreciated if this patch could be considered for merging as it would help us in research into performance of PQC algorithms; I admit that the extra lock/unlock operation around updates of the statistic adds overhead, I didn't immediately see an easy way to mitigate this as information on whether or not extended statistics are enabled is not available at the abstraction level where the statistic needs to be updated. It seems this is deemed acceptable for tracking the number of bogus RRsets, so hoping one more lock/unlock is fine here.