implement Array/Vector for serialization

It could be something like this: https://github.com/ChainSafe/ssz/blob/master/src/types/composite/array.ts

```
export class UnboundedArray extends CompositeType {
constructor(opts: {type}) {
}

serialize() {
//impl
}

deserialize() {
//impl
}

toJson() {
//impl
}

fromJson() {
//impl
}

}
```
and use it instead of ListType in `networkMetrics.ts`

_Originally posted by @mpetrunic in https://github.com/NodeFactoryIo/ChainGuardian/pull/327#discussion_r537355652_