PendingBlock + Blobs module API enhancement

[rodrigo-o]

There are some not-so-straightforward interactions between BlobSideCar calls PendingBlocks with added complexity due to the need of passing a callback to `

# lib/lambda_ethereum_consensus/p2p/gossip/blob_sidecar.ex
PendingBlocks.process_blobs(store, {:ok, [blob]})

# lib/lambda_ethereum_consensus/beacon/pending_blocks.ex
BlobDownloader.request_blobs_by_root(
    missing_blobs,
    &process_blobs/2,
    @download_retries
  )

...

def process_blobs(store, {:ok, blobs}) do

We need to rework the API to be more polished. We want to be able to let pending block know that a blob was downloaded without using an API that appears to point to other purpose.