feat: checkpoint verification

[matthias-wright]

This adds checkpoint verification.
Starting from the genesis, we verify that every validator set attests to the validator set of the next epoch. We also verify that the last validator set attests to the checkpoint hash of the loaded checkpoint.

Currently the --checkpoint-path CLI arg can be either a path to a checkpoint or a path to a checkpoint directory.
This format is kept, and the checkpoint directory is extended to house the finalized headers that contain the aggregated validator signatures:

checkpoint_dir/
├── checkpoint              # Required: SSZ-encoded Checkpoint
├── last_block              # Optional: SSZ-encoded last Block
├── finalized_header        # Optional: SSZ-encoded FinalizedHeader for the checkpoint epoch
└── finalized_headers/      # Optional: directory of epoch-indexed headers for verification
    ├── 0                   # SSZ-encoded FinalizedHeader for epoch 0
    ├── 1                   # SSZ-encoded FinalizedHeader for epoch 1
    ├── 2                   # ...
    └── n                   # SSZ-encoded FinalizedHeader for epoch n (the checkpoint epoch)

Changes:

• Implements checkpoint verification
• Adds an RPC endpoint for the finalized headers
• Adds e2e tests for checkpoint verification
• Updates checkpointing docs