allow sync using light client protocol

[etan-status]

Adds the --trust-sync-committee launch option to use the light client sync protocol for obtaining the latest finalized head. Then, instead of regularly syncing forward, sync backwards to skip most block validation. --trusted-block-root can be used to initialize the light client; when not specified, regular sync is used until the light client obtains info about new sync committees from the network to jump to latest wall slot.