Having a single import block entrypoint

[EclesioMeloJunior]

Issue summary

• Currently Gossamer has 2 main points where a block is imported, if the block is acquired through the network then we start the importing in the sync package, do the validations, execution and then pass it to core.HandleBlockImport to finish the block import handling block consensus messages and applying forced changes if needed. However, if the block was produced by the node the import is partially done in the babe package and then passed to core.HandleProducedBlock
• What I'm proposing here is to have a single point to handle/import blocks no matter how we acquire them, that way we can add a pipeline of checks and modifications (for example check if epochs were skipped) without repeating the logic in different places.
• In Substrate, this approach is used, also while importing the block is passed a set of parameters that help the import function to configure the import pipeline properly to a given block.

func (...) Import(block *types.Block, importParams *BlockImportParams) error {
  ...
}

[EclesioMeloJunior]

Turning this issue into an epic given that we need to conduct research on how to implement it, also on how the refactor should be done