Support key exists from TTL/PTTL command

When sending (P)TTL command to redis, the server return the ttl of a key, with two special values:

• -1 if the key has no expiry
• -2 if the key does not exists (added in redis 2.8)

https://redis.io/commands/ttl/ & https://redis.io/commands/pttl/

In the library, all negative vvalues are handle the same, and the fact the key does not exists is not handled
https://github.com/StackExchange/StackExchange.Redis/blob/main/src/StackExchange.Redis/ResultProcessor.cs#L318

This means we need to do two calls to redis to check if a key does not have an expire and know if it exists.

I would happily try to post a PR to add this behavior, but not sure on the api change: the library method KeyTimeToLiveAsync currently only returns one TimeSpan

Would adding a new method KeyExistsWithTimeToLiveAsync, returning a struct KeyExistsWithTimeToLiveResult containing a bool and a nullable TimeSpan be ok, adding a RedisFeatures KeyExistsExpiry (>=2.8), and a back up to calling both (P)TTL and EXISTS if feature not available?

Start with the TTL result handling in src/StackExchange.Redis/ResultProcessor.cs and the KeyTimeToLiveAsync API in src/StackExchange.Redis/RedisDatabase.cs. Review RedisFeatures and the Redis 2.8 TTL/PTTL semantics before deciding how the API should represent expiry and existence. Done means distinguishing -1 from -2 while preserving a fallback for older servers.