Add support for LightClientFinalityUpdate

[srdtrk]

Summary

Currently, our light client only supports LightClientUpdate. This type of update can currently only be used to update the next sync committee when moving in-between sync committee periods. We should also add support for LightClientFinalityUpdate for finality updates within the same sync committee period.

Problem Definition

Although LightClientUpdate could also be used update the finalized header within the same sync committee period, the ethereum full node's beacon api only returns the earliest such update. This essentially makes these updates unusable when updating the finalized header within the same sync committee period. For this, we can use LightClientFinalityUpdate, since there is an api's to receive the latest such update.

Use cases

We need this to have the light client be usable in testnet and mainnet.

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For Admin Use

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