PowerShell: .xaml file extension not supported

[dtivel]

Describe the bug
Sign CLI does not support signing PowerShell XAML files. This shows the file extensions for which PowerShell verifies signatures. Attempting to sign a simple XAML file results in HRESULT 0x800b0003 (TRUST_E_SUBJECT_FORM_UNKNOWN), which indicates that the form of the subject is not recognized. In this context, it's because the file format is not supported.

Repro steps

1. Add .xaml to AzureSignToolSigner's supported file extensions.

2. Save the following content to file user-interface.xaml.

   <Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
           Title="Hello World" Height="200" Width="300">
       <Grid>
           <Button Name="HelloButton" Width="100" Height="30" VerticalAlignment="Center" HorizontalAlignment="Center" Content="Click Me"/>
       </Grid>
   </Window>

3. Try to sign the file.

Expected behavior
Signing succeeds.

Actual behavior
Signing fails.