Need API to load state snapshots with on-disk MerkleDb indices

When a state snapshot is loaded into memory, MerkleDb uses off-heap DB indices by default. There is a way to use on-disk indices, but it isn't exposed as API. This functionality, on-disk indices, will be of great value for block nodes, since they may need to load multiple snapshots into memory at the same time.

Start by locating the state-snapshot loading path and MerkleDb's existing on-disk index configuration. Define how the loading API should expose that option, then verify that multiple snapshots can be loaded with on-disk indices as requested.

Written by the indexing model from the issue text.