Change GossipRpcShadowgraphSynchronizer construction from network layer to platform

As part of separation of concerns between network and sync layer, we want to construct GossipRpcShadowgraphSynchronizer outside of network code. This depends on #19763 and #19764 being done, so we don't have shared dependencies anymore.

Start by reviewing GossipRpcShadowgraphSynchronizer and the separation-of-concerns goal described in this issue. Check blockers #19763 and #19764 first to confirm the shared dependencies are removed. Done means the synchronizer is constructed outside the network layer without those shared dependencies.

Written by the indexing model from the issue text.