DRILL-7547: Support credentials store for mongo connections
This uses the hadoop `Configuration.getPassword` method to retrieve the
username and password for mongo connections. This allows the user to
supply credentials or credential store configuration in core-site.xml
instead of inlining the credentials in the storage plugin configuration
that is stored in ZooKeeper.
Refer to the CredentialProviderAPI document for more information about
how credential provider plugins work.
closes #2001
