dsc_credential should accept string

[Clebam]

This PR changed string to senstive string. However it complexifies a lot if puppet is used with hiera (and the migration from old puppet dsc to pwshlib is even harder)
https://github.com/puppetlabs/Puppet.Dsc/pull/19/files

A suggested solution would be to use a variant as here : https://github.com/voxpupuli/puppet-mongodb/blob/master/lib/puppet/functions/mongodb_password.rb#L9

'Variant[String[1], Sensitive[String[1]]]'

This would allow to use string or sensitive string