Add TLS example with "production-ready" options.CertificateValidation and test it in CI against Redis Cloud

[uglide]

Since TLS support of Redis cloud is skipped in the SE.Redis, we need to test it in this repo and provide "production-ready" example of how to validate TLS certificates for Redis Cloud in options.CertificateValidation

[slorello89]

@uglide - the appveyor tests for StackExchange.Redis are testing on windows servers against a version of Redis 3.1 built to run on Windows servers, hence they are not a valid gauge for whether it's running the thing or not, in the days of old GitHub actions did not support Windows VMs hence the bifurcation in builds. We did extensively test TLS capabilities as part of StackExchange/StackExchange.Redis#2219. the CertificateValidation workflow is somewhat obsolete with the introduction of SslClientAuthenticationOptions in .NET Core 3.1 (See RU102N Redis for .NET Developers Section 1.4.2), a delegate for which was which was added at the time to StackExchange.Redis.

Is there a definitive need for this to be tested against Redis Cloud or would it be sufficient to test it against. a TLS Enabled version of Redis as it's done in StackExchange.Redis. Do we want to add a hard dependency for CI on an external service? We were trying to avoid exactly this two weeks ago to do all the testing for .NET Framework.

[uglide]

@slorello89 Thanks for pointing it out. For me, it sounds like we should provide two examples for .NET 4 with options.CertificateValidation and .NET 5+ with SslClientAuthenticationOptions instead of insecure return true; like here https://github.com/StackExchange/StackExchange.Redis/blob/9698aaa8c5e372a73e0dec0adeeb9c85c09dd7f9/tests/StackExchange.Redis.Tests/SSLTests.cs#L207

We should test end-to-end with Redis cloud to guarantee the best developer experience. I don't know what to add here or how to achieve it without dependency on real Redis Cloud with TLS.