Implement ZADD INCR flag

[mgravell]

Missing 3.0.2 feature.

See https://stackoverflow.com/questions/79935882/stackexchange-redis-implementing-zincrby-with-xx-flag and https://redis.io/docs/latest/commands/zadd/

[mgravell]

The API shape makes this inappropriate for SortedSetAdd / SortedSetUpdate; I propose to make this SortedSetIncrement when a "when" flag; usage:

var value = await db.SortedSetIncrementAwait(key, member, 12, when: SortedSetWhen.Exists);
if (value is not null)
{
    Console.WriteLine("Pre-condition failed, shucks");
}
else
{
    Console.WriteLine("New value: {value}");
}