refactor: decouple snapshot-interval from state-sync

[p0mvn]

Context

#140 , #187 refactored snapshot logic.

In these PRs, it was done so that the snapshot manager is not set (nil) if snapshot-interval is 0. However, snapshot manager is also responsible for state-sync. It might be the case that a node wants to catch up using state sync while turning off snapshots on their side.

Logs with this issue:

3:51AM INF Discovered new snapshot format=1 hash="\r��z��ܓ\x18Ws��\x19d�=ެ��=�\n\x05��\v���m" height=4329000 module=statesync
3:51AM INF Discovered new snapshot format=1 hash="\x008\x05(W���Y\b�b&��jڭ�\x10�,\a��o\x04c\a�E4" height=4327500 module=statesync
3:52AM INF VerifyHeader hash=6E992F698F0D454B2AC5458CB1C467BF9B393A11C86025CA986DA1235DF23238 height=4329001 module=light
3:52AM INF VerifyHeader hash=98CA996C1ED5ECA1731CEACA67EE08B03755C690F38745E50146AFAF2455360B height=4329002 module=light
3:52AM INF Offering snapshot to ABCI app format=1 hash="\r��z��ܓ\x18Ws��\x19d�=ެ��=�\n\x05��\v���m" height=4329000 module=statesync
3:46AM ERR snapshot manager not configured
3:46AM ERR State sync failed err="state sync aborted" module=statesync

We should decouple the logic so that the snapshot manager is created independently from the snapshot-inteval value in app.toml

Acceptance Criteria

• state sync is enabled independently from the value of snapshot-interval in app.toml
• unit tests added
• snapshots and their configuration works as expected