Implement `BeaconNetwork` spec updates

[acolytec3]

From this PR, we need to make a few updates to our BeaconNetwork implementation:

• When handling a FINDCONTENT request for LightClientUpdatesByRange, respond with a subset of updates if we have a consecutive set (even if not the the entire requested range)
• When handling FINDCONTENT requests for OptimisticUpdate and FinalityUpdate, ensure we check if the slot we have an update for is >= the one requested and send if so. Otherwise, send empty response.