[Raft] Add generic raftstore

[MariemBaccari]

Implements the generic raftstore which processes and applies committed entries.

[mickmis]

Not sure I understand why GetRepo() is needed? This makes the struct return itself. Why not taking the struct directly in the first place?

[MariemBaccari]

I attempted to remove it in this commit but at the cost of a not-so-pretty type assertion repo, ok := r.(R). Let me know what you think / if you have a better or more idiomatic alternative.

[mickmis]

OK I now see the issue, indeed the assertion is not nice, your previous solution is better.
Alternatively we could also have Init accept 2 arguments (MemRepo and R) but that's roughly equivalent, so up to you.

Actually it would have been nice to be able to do the following, however that's not legal in Go:

type MemRepo[R any] interface {
	T
	GetSnapshot() ([]byte, error)
	RestoreFromSnapshot([]byte) error
	Checkpoint()
	Restore()
}

[mickmis]

LGTM modulo comment + revert or implementation of alternative for MemStore

Taking liberty to dismiss review as @barroco is away and I believe all of his comments were adressed.