Fix blob download from checkpointz servers

[michaelsproul]

Issue Addressed

Closes:

• Add fork version fallback for GET /eth/v1/beacon/blobs #7489

Proposed Changes

Use ForkName::latest_stable, i.e. Electra when decoding blobs from a server that does not provide version. This is only a temporary workaround that should be reverted once checkpointz is fixed. Having a default fork is potentially incorrect, and glossing over bugs in servers in general is not ideal.

However, even in the case where we update ForkName::latest_stable to Fulu, this should continue to work, as the blob limit is likely to increase and the RuntimeVariableList will just have a slightly higher limit than necessary (which is OK so long as the server isn't buggy enough to violate the correct lower bound: e.g. if the block is an Electra one and the server sends 10 blobs, which exceeds the Electra max (9) but not the Fulu max).

Additional Info

I've opened an issue to revert this PR before we proceed with Fulu on mainnet, but after v7.1.0:

• Revert fork version fallback used by /eth/v1/beacon/blob_sidecars/{block_id} #7496

[mergify]

Some required checks have failed. Could you please take a look @michaelsproul? 🙏