[OB4] Implement structures to store candidate chains for selection [M2]

[creativcoder]

Issue summary

• The NCN will get the best block from the pool of trusted providers, it will run chain selection and select the best block,
• It will track n blocks behind that best block for stronger finality purposes (where n may = 20 (an arbitrary number) or the same number of blocks tracked by exchanges.)
• As a user I can specify how many blocks in history that the NCN will store so that I have flexibility over the finality confidence of my node
• Integration tests

Other information and links

[creativcoder]

block on: usecase of finality check and the data structure API.