Clean up and document Snowflake client package

This is the first of a series of changes to the Snowflake library in advance of a version 2 bump.

This MR includes changes to the items exported by the client library, and documentation fixes to meet best practices. There aren't a lot of functional changes here other than to remove some unused code, make some functions and variables internal to the package, and export the RendezvousMethod interface to allow users of the package to specify their own rendezvous methods with the broker.

Part of #40063 (closed)