feat(beacon): add full validation for light client updates

[ogenev]

What was wrong?

Fixes #1406

This PR adds full light client content validation for LightlClientUpdate, LightClientOptimisticUpdate, and LightClientFinalityUpdate content on a protocol level. (before storing and gossiping the content to peers).

How was it fixed?

• Add a new JSON-RPC endpoint to get the current LIghtClientStore data.
• Refactor and expose the validation method as a public from the light client module. This allows us to use the same validation inBeaconValidator as the light client sync protocol.
• Add the validation steps in BeaconValidator.

To-Do

• [ ] Clean up commit history and use conventional commits.