SNOW-576663: Allow using a private key stored in AWS KMS for key pair auth

[meastham]

We would like to use a private key stored in AWS KMS for authenticating using they Snowflake key pair auth mechanism. The current JDBC driver, which requires the actual content of the private key to be present in a local file. This isn't compatible with KMS, which never exposes the private key, but rather gives you access to a signing API.

It seems to me a straightforward way to accomplish this is to allow the library to be configured with a class name for a custom signing implementation. I will follow up shortly with a PR implementing this proposal.