Align class names and code structure with the connectors in other languages #341

The connectors in the other languages are structured with common names and separation of concerns.

Public API:

• Connector is the main public API to configure and connect.
• ConnectionOptions is the value object containing connection options. main public API to configure and connect.

Internal API:

• ConnectorRegistry is the internal class that holds configuration and caches for each database instance.
• ConnectionInfo is a value object that holds the configuration and data for one database instance.
• ConnectionInfoCache interface holds the active ConnectionInfo value for a single database instance and keeps it up to date, refreshing the data as needed.
• RefreshAheadCache implements the ConnectionInfoCache using the current connector refresh algorithm
• LazyRefreshCache (to be added with Add support for lazy certificate refresh #285) implements ConnectionInfoCache using the lazy refresh algorithm.
• ConnectionInfoRepository calls the Cloud SQL Admin API to build an up-to-date ConnectionInfo object.

To get to the NodeJS connector codebase into state, we will need to move some code and rename some classes.

• Rename CloudSQLInstanceMap to ConnectorRegistry
• Rename InstanceConnectionInfo to ConnectionInfo
• Rename CloudSQLInstance class to RefreshAheadCache
• Rename Fetcher interface to ConnectionInfoRepository
• Rename SqlAdminFetcher class to DefaultConnectionInfoRepository