CircMgr: Add an accessor for the CircuitBuilder.

Without this, actually building circuits manually is a pain.

This API is behind the experimental-api feature, and so it does not require a semver.md entry.

assigned to @nickm

cc @juga

requested review from @Diziet

merged

mentioned in commit 6b7b4896

mentioned in issue tpo/network-health/margot#4 (closed)