SL-11216: Add llsd::drill() function to drill into an LLSD blob.

We include both const and non-const overloads. The latter returns LLSD&, so
you can assign to the located element.

In fact we already implemented the non-const logic in a less public form as
storeToLLSDPath() in lleventcoro.cpp. Reimplement the latter to use the new
llsd::drill() function.