`proposer_pubkey` is missing from schema and examples

[ralexstokes]

it looks like the flashbots/mev-boost-relay expects the proposer's public key:

https://github.com/attestantio/go-builder-client/blob/master/api/v1/bidtrace.go#L25

and these specs omit it.

this makes it harder for others to use the specs to contribute to the ecosystem and we should correct it :)