Avoid cast exception

[stheppi]

CREDENTIALS property was set as STRING and is read as a PASSWORD. This creates a runtime exception. The change moves the connector configuration to be a password.

java.lang.ClassCastException: class java.lang.String cannot be cast to class org.apache.kafka.common.config.types.Password (java.lang.String is in module java.base of loader 'bootstrap'; org.apache.kafka.common.config.types.Password is in unnamed module of loader 'app')
	at org.apache.kafka.common.config.AbstractConfig.getPassword(AbstractConfig.java:216)
	at io.lenses.streamreactor.connect.gcp.storage.config.AuthModeSettings.$anonfun$getAuthMode$3(AuthModeSettings.scala:99)
	at scala.util.Try$.apply(Try.scala:210)