Introduce an unexported newBrokerChannelFromConfig

A follow-up wants to pass in a new property from the ClientConfig but it
would be an API breaking change to NewBrokerChannel.

However, it's unclear why NewBrokerChannel is exported at all.  No other
package in the repo depends on it and the known users of the library
probably wouldn't be construct them.

While this patch was being reviewed, a new constructor was added,
NewBrokerChannelWithUTLSSettings, with effectively the same issue.
Both of those exported ones are deleted here.

client/lib/snowflake.go

@@ -131,9 +131,7 @@ func NewSnowflakeClient(config ClientConfig) (*Transport, error) {
 	}
 
 	// Rendezvous with broker using the given parameters.
-	broker, err := NewBrokerChannelWithUTLSSettings(
-		config.BrokerURL, config.AmpCacheURL, config.FrontDomain,
-		config.KeepLocalAddresses, config.UTLSClientID, config.UTLSRemoveSNI)
+	broker, err := newBrokerChannelFromConfig(config)
 	if err != nil {
 		return nil, err
 	}