Make Title and Message settable in Credential Attribute

For parity with Get-Credential, this exposes Title and Message properties in the Credential attribute, while maintaining the current behavior.

Requested by @potatoqualitee ...

https://twitter.com/cl/status/1339896630863159296

The key feature request was to be able to customize the prompt for the password even when a username was being passed to the Credential parameter.

Note: I've not touched any tests here. If anyone who thinks something more should be done can point me in the right direction, I'm happy to do it.

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