MVP impl of `import-blobs`

[michaelsproul]

Issue Addressed

Closes:

• https://github.com/sigp/lighthouse/issues/5391

This is just a quick interim solution to allow blobs to be imported from an existing Lighthouse DB. We will continue working on a more general solution.

Proposed Changes

New commands:

• lighthouse db import-blobs --source-db /path/to/blobs_db: import blobs from another node.
• lighthouse db inspect-blobs: iterate all blobs in the node's DB and look for gaps.
• lighthouse db set-oldest-blob-slot: manually hack the oldest_blob_slot (shouldn't be necessary if using import-blobs).

[michaelsproul]

Recommended usage:

./lighthouse db import-blobs --datadir ./datadir/ --source-db ./blobs_to_import --debug-level debug

Running as the user with the correct permissions for both ./datadir and ./blobs_to_import