Add the ability to provide a custom prompt when building SwiftUI VMDTextField

[dominic-plouffe]

Description

Add a new initializer to VMDTextField allowing to customize the placeholder.

Motivation and Context

Since iOS 15, TextField provides a constructor where we can pass a Text as the prompt (placeholder).

However, the implementation of VMDTextField passes a Text(viewModel.placeholder), preventing the customization of the prompt (placeholder).

How Has This Been Tested?

Added custom foreground color to the sample

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)