Blind the bloom filter in `getHeader` response

[ralexstokes]

Combination of #108 and #120.

Instead of authenticating the getHeader endpoint as in #108, and instead of removing as much data as possible as in #120, we can instead patch the issue described in #108 by blinding the bloom filter in the getHeader response.

To do so, we would replace the logs_bloom field in the response with the root hash of that SSZ field. This still enables the proposer to assemble the correct signature which they can return to the relay as proof that they have accepted the builder's bid.