Add new endpoint to support downloading of blobs (EIP-4844)

[jimmygchen]

Raised by @ralexstokes in this PR #271

The above PR adds new types to support EIP-4844l, however there is currently no endpoint for users to download blobs from their local CL nodes.

@0xGabi suggested the following:

I think we should only allow to download a list of blobs_sidecar using the block_id identifier. For example: /eth/v1/beacon/blobs_sidecar/{block_id}. Downloading a single blob could be handle with a higher level library and reduce CL complexity.

[mcdee]

Why v2 not v1?

[0xGabi]

It was a mistake on my side. I copied one example from the beacon-api website and didn't realize it was a v2 example. We have to start with v1 👍

[jimmygchen]

Thanks, updated the description :)

[jimmygchen]

I have implemented this endpoint in Lighthouse, PR here. Will pick up the Beacon API spec early next week.

[jimmygchen]

Completed in #286