[Consensus Observer] Add state sync fallback mode.

[JoshLind]

Note: this PR is built on https://github.com/aptos-labs/aptos-core/pull/14954

Description

This PR adds a "state sync fallback" mode to consensus observer (CO). If CO does not make progress for some time (e.g., no increase in synced version for 30 seconds), it will fallback to state sync for a while (e.g., 10 minutes). After which, CO will regain control and attempt to make progress.

The PR offers the following commits:

1. Rename sync_to() to sync_to_target()
2. Add the state sync fallback mode to CO.
3. Enable CO for all validators and VFNs (by default).

Testing Plan

New and existing test infrastructure, including manual verification.