expose Prepend and Append methods on RedisKey, so we can avoid some string concats in common cases

We've got some code that routinely concats an arbitrary string to a fixed prefix.  It'd be nice to not have to allocate a whole new `string` (or `byte[]`, or whatever) when we're just gonna turn this into a `RedisKey` and be done with it.

This commit exposes the already existing [`WithPrefix`](https://github.com/StackExchange/StackExchange.Redis/blob/d994b7ad57a76586487cd2d460d55cb850498448/StackExchange.Redis/StackExchange/Redis/RedisKey.cs#L253) method on `RedisKey` in separate `Prepend` and `Append` wrapper methods.