chore: Set default negative starting blocks on the example configuration

[jsanmigimeno]

Consider setting default negative starting blocks on the example configuration. This will allow passed BountyPlaced events (and similar) to be processed, and will allow proofs received right after initialization to be submitted, rather than only being able to process new bounties that are registered from the time at which the Relayer is initiated. Do note this may cause to have to use archival nodes for the rpcs.