Create `Head` method

[tzdybal]

Head method (corresponding to /head endpoint) needs to be added to client.

Specs: https://docs.celestia.org/developers/node-api/#get-head Code in node: https://github.com/celestiaorg/celestia-node/blob/0466d9c9222f7ceacba2223a83fd352b5d751d1e/service/rpc/header.go#L22

This is height-less version of /header endpoint.