Validate Realm account in accounts context

[armaniferrante]

If we add a Realm deserializer to the anchor_spl crate, we can move this check to the #[derive(Accounts)] context.

Low priority but would be nice to not use an UncheckedAccount.

[armaniferrante]

One annoying thing about this is that, because there are many gov program ids, we need to generate a new Realm type for each gov program like we do with vote_weight_record!, so that it has the correct owner pubkey.