chore(pkg/finality-grandpa): Implement `saturating_add` and `saturating_sub` on `VoteWeight`

[timwu20]

Issue summary

• The rust code has some macros which generate the saturating_add and saturating_sub methods for certain types to avoid overflows.
• We should implement this as well for VoteWeight in the go package

Other information and links

• https://github.com/paritytech/finality-grandpa/blob/a5bd965782216a95f017a65527a0a4fe28203e4d/src/weights.rs#L38

[EclesioMeloJunior]

• [x] Implement saturating_add and saturating_sub (https://github.com/ChainSafe/gossamer/pull/3519)
• [ ] Use saturating_add and saturating_sub on VoteWeight