add beacon_node_light_client_data_* variables

[etan-status]

To control the --serve-light-client-data and --import-light-client-data flags.

Note: These new flags are only available on unstable at this time. stable does not support them yet.

[jakubgs]

Wait, what format data does the --import-light-client-data= flag expect?

[etan-status]

https://github.com/status-im/nimbus-eth2/blob/unstable/beacon_chain/conf.nim#L438

One of:

• none
• only-new
• full
• on-demand

It's only on unstable so far, not yet available on testing or stable.

[etan-status]

Yep, or we can also just wait until the new flags have propagated far enough. Will update this again once that is the case.

[etan-status]

• Wrapped in a conditional

[etan-status]

• Use common prefix beacon_node_light_client_data_ for new variables.