feat: add block signature validation

[hal3e]

closes: #57

This PR introduces a new validator crate which holds a BlockValidator struct that can validate blocks coming from the fuel network. Once the block_watcher receives a new block it is validated by the BlockValidator and a new ValidatedFuelBlock is then propagated throughout the block committer.

[segfault-magnet]

Super clean! Have one thought I'd like to share:

Wdyt about creating our own fuel block type instead of just reexporting from fuel_core_client for the following reasons:

• We can write fuzzing/parameterized tests for the block validation (since we can derive stuff for the block type)
• We can remove the fuel_core_client dep from the ports

or just write tests for every hash input.

Mostly because of 1. since there is so much stuff that goes into the hash, we should probably cover all of it with tests -- i.e. test that any change in any of the inputs should render the block invalid since this is so sensitive in nature.

Ideally we'd do some form of fuzzing or something akin to it just so we cover as much as possible.

[Br1ght0ne]

LGTM, having read through the code. I'll not pass judgement as I don't have enough committer knowledge.