syncer: allow setting sync target or network head manually

Instead of relying on automatic subjective initialization over `trustedPeers`, infrastructure providers may want to run their nodes with a custom target addressed with a hash.

Start in the syncer initialization path, especially where automatic subjective initialization uses trustedPeers. Trace how a custom target or network head could be supplied by hash, and identify the existing syncer tests or entry points that cover initialization. Done means infrastructure providers can set the target manually without relying on trustedPeers.

Written by the indexing model from the issue text.