Motivation:

While working through issue #59, it was noticed just how "stringly" the SortedSet command options for `zadd`, `zinterstore`, and `zunionstore` were, and Swift provides ways of having strong type safety for these options.

Modifications:

- Add `RedisSortedSetAddOption` and `RedisSortedSetAggregateMethod` to replace the String API in `zadd`, `zinterstore`, and `zunionstore`
- Fix an implication of how `overestimatedCountBeingAdded` documentation for `Array where Element == RESPValue` for `add(contentsOf:overestimatedCountBeingAdded:_:)`

Result:

Users should have a more discoverable and straightforward way that isn't error prone for calling `zadd`, `zinterstore`, and `zunionstore` with Redis supported options.