Optimizer Trace replay step #3: load basic statistics back

Continuation of MDEV-36511.
Assuming the user has extracted the JSON section from MDEV-36511 and has provided its contents in a variable, create code to parse it back.

Then, use the hooks to replace the statistics fed into the optimizer.

Note: one can use a storage engine (i.e. an object derived from class handler) to capture or fake the statistics, but is this a good approach? Inheriting from class handler (or, e.g. ha_innobase) seems to be quite complex.