Make it easier to use user certificate files

[mgravell]

Historically, certs only worked with PFX files, which made it a pain to work with the cert pairs typically downloaded from hosts like Redis Ltd; however, on more recent .NET versions PEM is fully available. Here, we:

• add new methods on ConfigurationOptions (akin to the existing TrustIssuer method) to configure certificates from files
• support PEM in the ambient setup

There are no tests added here, due to the inherent problems of CI talking to such servers; however:

 var options = ConfigurationOptions.Parse("redis-redacted.redislabs.com:4242"); // port also redacted
 options.TrustIssuer(@"C:\Code\RedisKeys\redis_ca.pem");
 options.SetUserPemCertificate(@"C:\Code\RedisKeys\redis-redacted.crt", @"C:\Code\RedisKeys\redis-redacted.key");
 using var conn = await ConnectionMultiplexer.ConnectAsync(options);
 for (int i = 0; i < 10; i++)
 {
     var ttl = await conn.GetDatabase().PingAsync();
     Console.WriteLine(ttl);
 }

works with the files downloaded from my hosted Redis Ltd endpoint:

To @philon-msft : open question: can we check Azure Redis with similar?

^^^ update: Philo assures me that Azure Redis doesn't have any relevant client-cert scenarios to consider

[mgravell]

In particular: contrast this to the recommended approach currently on my hosted Redis Ltd instance:

 configurationOptions.CertificateSelection += delegate
        {
            return new X509Certificate2(
                "<path_to_redis.pfx_file>",
                "<pfx_file_password>"
            );
        };

Which a: requires additional manual steps to export the pem/key file to pfx, which isn't trivial, and b: is more additional work. Sure: if you want more control, it is great to have the full options, but most people will just have the file pair.

[mgravell]

also /cc @atakavci to a: see if there's any feedback from the Redis side, and b: (future, obvs) see if we can get updating the cloud console updated to suggest this approach.

[NickCraver]

One quick comment - TODOs: get test server setup and docs here as follow-ups

[NickCraver]

Could use .TryParse a bit simpler here

[mgravell]

done: 7a9208f