Add x509storeadd perf test

[npajkovsky]

Measure X509_STORE_add_cert()/X509_STORE_add_crl() as the store grows. The store keys objects by their subject/issuer X509_NAME into a hash table, so the cost of an add depends on how the names distribute over the buckets. Two workloads are measured across a sweep of store sizes (default 128..32768, override with -n):

spread - every object carries a distinct name and lands in a bucket
of its own, with randomised insertion order; the O(1)
common case.

bucket - every object carries the same name, so all of them collide
into a single bucket whose growth dominates the add cost;
the pathological case (e.g. many cross-signed certs sharing
one subject).

Both workloads insert a mix of programmatically generated certificates and CRLs signed with a throwaway Ed25519 key. For each size N the test fills a fresh X509_STORE with N objects, timing only the add loop and repeating until enough time has accumulated for the ~1us clock resolution; the best ns/add of 5 trials is reported.

With -d the results are also written to a gnuplot data file (columns: N spread bucket, NaN for a mode not measured; use -m both to fill all columns) and -l records a build label in the header, so results from different builds can be plotted against each other with the included x509storeadd.gnuplot script.

The tool is single-threaded and not wired into the cmake run target; build it and run on demand. Requires OpenSSL 3.0+ (EVP_PKEY_Q_keygen), so it is not built for 1.1.1 or the OpenSSL forks.

Assisted-by: Claude:claude-fable-5