sync: Add a boolean status to Syncer struct for tracking started state

The Syncer status can keep track of whether it has started already or not in a boolean.

It can check it in its `Start` and `Stop` method. If the status is started at the beginning of `Start` or `Stop`, the methods should be a no-op and can just return `nil`.

Otherwise, if we wish to not change `Start` or `Stop`, we can just add a getter for this status.

Rollkit uses the `go-header` library to implement `HeaderExchangeService` and it has to make a `SyncerStatus` struct to keep track of this state. Code [here](https://github.com/rollkit/rollkit/blob/1e1d8f9d8f5fe71fd5d7f0790408bbaf574dee80/node/header_exchange.go#L42).

Adding this to the `Syncer` struct would allow Rollkit to eliminate using the `SyncerStatus`.

Start at the Syncer struct and inspect its Start and Stop methods to understand their current lifecycle behavior. Decide whether the started status should make repeated calls no-ops or be exposed through a getter, then verify that the chosen behavior lets Rollkit avoid its separate SyncerStatus state.

Written by the indexing model from the issue text.