Here we add a beacon chain light client for use by geth.
Geth can now be configured to run against a beacon chain API endpoint,
without pointing a CL to it. To set this up, use the `--beacon.api` flag. Information
provided by the beacon chain is verified, i.e. geth does not blindly trust the beacon
API endpoint in this mode. The root of trust are the beacon chain 'sync committees'.
The configured beacon API endpoint must provide light client data. At this time, only
Lodestar and Nimbus provide the necessary APIs.
There is also a standalone tool, cmd/blsync, which uses the beacon chain light client
to drive any EL implementation via its engine API.
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Co-authored-by: Felix Lange <fjl@twurst.com>
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