Support for Specifying SSL Cert

[bobcat1506]

Add support for specifying an SSL cert to use when connecting to Mongo.

Perhaps more broadly allow configuration of MongoClientSettings or allow the application to create the MongoClient instance via a callback method.

[ricksearcy]

This is currently a deal breaker as we have a requirement to only use Tls connections to mongo. Are there any plans to support this in the near future? Would you accept a PR if I added in support for this?