lib(pkg/finality-grandpa): standalone finality-grandpa package

[timwu20]

Changes

• standalone finality-grandpa package ported from analogous rust implementation
• includes tests
• largely reviewed by @jimjbrettj on feat/grandpa branch (see lib(pkg): standalone finality-grandpa package #3235)

Tests

go test -tags integration github.com/ChainSafe/gossamer

Issues

contributes to #3468

[kishansagathiya]

Not a fan of heavy use of generics, wonder if there is a simpler way

[jimjbrettj]

My opinion on the use of generics here (since I see it discussed above).

Currently, I tend to lean away from their use in this package as it does not seem necessary to me and I don't really think we should use them unless there is good reason. However, it is a package so generics can make sense in this context, even if the immediate usefulness isn't there from my pov. So I can understand this.

The other point I wanted to make is that the merging of this PR does not mean we can never modify this code again. The main reason this is being merged right now, to begin with, is to help support another PR. So I think discussions around generics, code structure, and other larger discussions that have implications outside of just this PR should be had, but perhaps they are more ongoing conversations rather than explicitly tied to this PR.

To be clear, I am fully in favor of discussing these points. I just want to make sure that the context that this PR exists in is considered when deciding if large changes should be prioritized in this PR or handled by future work.

[jimjbrettj]

As noted, I have already reviewed this work separately. I still gave it another look over, added a few comments, and contributed to a few ongoing discussions.

This is really awesome work. A ton of work went into this and I am stoked to see it being merged, LGTM!

[EclesioMeloJunior]

Already reviewed this PR some time ago