New event to signal to builder that bid was accepted?

Based on some discussion I had with @rolfyone we might need an event for gloas that allows a CL client that "acts as a builder" to know whether the bid was accepted and included in a block by a proposer without having to subscribe to gossip, or alternatively listen to `block` event and call `getBlockV2` to inspect the contents. A simple fix would be to include the builder index in the `block` event but that requires adding a field, which should be no issue but need to see if people are happy with this.

Start by reading the issue discussion and the beacon-APIs specification around the `block` event and `getBlockV2`, with the GLOAS context in mind. Compare the proposed builder-index field with a separate event and document the agreed interface and completion criteria once the API behavior is decided.

Written by the indexing model from the issue text.