Endpoint for fetching blinded blocks

[michaelsproul]

This PR adds a new endpoint analogous to /eth/v2/beacon/blocks/{block_id} that serves blinded blocks rather than full blocks. The new endpoint is /eth/v1/beacon/blinded_blocks/{block_id}.

We already have other endpoints for creating new blinded blocks and POSTing signed blinded blocks back to the BN, so this PR completes the trilogy.

There are several reasons why such an endpoint is useful and desirable:

• Most consensus clients do not store full blocks with execution payloads in their databases. Fetching a finalized blinded block can be done more quickly and efficiently than serving a full block which has had its payload pruned and would require a call to the EL to reconstruct.
• Blinded blocks contain a lot of useful information that isn't in block headers, including the execution block hash, execution block number, attestations, exits, etc. For many consensus-oriented tools blinded blocks are sufficient to conduct their analysis (this is true of my block fingerprinting tools, and @jclapis's Rocket Pool tools). Blinded blocks can also be useful for builders and execution-oriented tools which might want to know e.g. the block_number of the head block, without fetching all transactions.

[rolfyone]

LGTM. should be easy to implement in teku, I know you were seeking opinions on that too...

[djrtwo]

looks reasonable

[mpetrunic]

Small comment, otherwise looks good!