Provide beacon node with proposals for the given validators.

Prepares the beacon node for potential proposers by supplying information
required when proposing blocks for the given validators. The information
supplied for each validator index will persist through the epoch in which
the call is submitted and for a further two epochs after that, or until the
beacon node restarts. It is expected that validator clients will send this
information periodically, for example each epoch, to ensure beacon nodes have
correct and timely fee recipient information.

Note that there is no guarantee that the beacon node will use the supplied fee
recipient when creating a block proposal, so on receipt of a proposed block the
validator should confirm that it finds the fee recipient within the block
acceptable before signing it.

Also note that requests containing currently inactive or unknown validator
indices will be accepted, as they may become active at a later epoch.

Click Try It! to start a request and see the response here!