Add standardized debug logging

[jackwotherspoon]

Add support for optional debug logging. Presently, we want to log the following:

Logging for refresh operations

• When a refresh operation starts
• Before the rate limiter is acquired
• After the rate limiter is acquired
• When a refresh operation finishes
• When a refresh operation errors
• When a refresh operation is canceled
• The ephemeral certificate’s expiration time
• The next scheduled refresh

Logging for connection attempts

• The current certificate’s expiration
• The result of the invalid certificate check (i.e., is “now” after the expiration)
• The result of connecting to the TCP socket including the IP address / PSC DNS name
• The result of the TLS handshake

Logging for connection cache

• Adding connection info to the cache
• Removing connection info from the cache

[jackwotherspoon]

Ref: GoogleCloudPlatform/cloud-sql-go-connector#726