add random walk endpoint with limited retries

Related to #1969.

For several "walk from point A to point B" use cases it doesn't matter which path is taken, only whether one exists. Hence we want to add an endpoint that performs random walks (also known as Bernoulli walks) on the graph, trying to find a destination that matches the desired specification (i.e., target node ID or target node type—as in other visits.

---

Migrated from T2077 (view on Phabricator)