feat(storage): make snapshots non-optional in provider factory

[shekhirin]

Snapshots are becoming the integral part of querying any data from Reth node (from both inside our codebase and outside using the external process DB access): we store headers, transactions and receipts in snapshots, and want to make the way users access data abstracted away from the notion of MDBX/static file storage.

This PR makes SnapshotProvider in ProviderFactory non-optional, meaning that to initialize a provider factory you'll need to specify a path to the snapshots directory.

[gakonst]

good w me would love if we could maybe always have the snapshots argument default to the value provided right now explicitly, since db & snapshots are under same datadir atm. but that might change in the future if mounting from a separate drive, i guess