Nakamoto Signer[3.0] - Update Block Header to use a vec of Signatures

[saralab]

stackslib: chainstate::nakamoto

The NakamotoBlockHeader struct will need to be updated to include a vec of signatures rather than an aggregated signature.

The block validation paths (append_block) and (accept_block) will need to be updated with the new signature validation logic.

[blockstack-devops]

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