cryptonote_basic: add overload for get_block_longhash()

[jeffro256]

Moves the core logic for get_block_longhash() into cryptonote_basic library from cryptonote_core. No change in behavior should occur, it's purely a refactoring.

Depends on #9919

[vtnerd]

These changes look acceptable - but why make the change? Is this related to fcmp++ in some way?

[jeffro256]

I intend for get_block_longhash() to be usable in BlockchainLMDB to perform database migrations for if we want to integrate #10038. If we don't, it's also just cleaner IMO.

[jeffro256]

Rebased to remove #9919 from commit list

[selsta]

@vtnerd please re-review