Add optimistic relaying and optimistic v2 endpoints to spec

[avalonche]

Currently many relays implement the optimistic relaying functionality which is not documented anywhere in the spec.

A high performance rust implementation of the boost relay has recently been open sourced which introduces two new endpoints /relay/v1/builder/headers and /relay/v1/builder/blocks_optimistic_v2. This separates the block submission process into two parts - the builder submits only a header first and then the rest of the payload to a separate endpoint to minimise network latency.

This endpoint should be standardised across relays implementing the optimistic v2 proposal which only parses the payload header and some other fields to construct the getHeader response.