fix(ze-api): The `ZoweVsCodeExtension.promptUserPass` should use placeholder values instead of hardcoded ones

[zFernand0]

I noticed there is text pre-filled in the input boxes for user/password. I think we want to retain the previous behavior of placeholder values that are "grayed out".

Before

After


(here I have not entered a password yet, but it is trying to show "Password" which renders as dots)

Originally posted by @t1m0thyj in #2606 (review)

[github-actions]

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