feat: support custom headers for Xray HTTP outbounds (#9690)

* feat: support custom headers for Xray HTTP outbounds

* feat: simplify HTTP outbound headers input

* feat: validate JSON syntax in HTTP headers

  Keep HTTP outbound headers stored as raw text, while validating
  header names and values against RFC 9110/9112 syntax. Support string
  arrays for multi-value headers, reject duplicate JSON header keys, and
  clarify the invalid header JSON message.

* feat: refactor HTTP header parsing into utility

* Simplify code

* Simplify code

* Update CoreConfigV2rayServiceTests.cs

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Co-authored-by: 2dust <31833384+2dust@users.noreply.github.com>

v2rayN/v2rayN.Desktop/Views/AddServerWindow.axaml

@@ -256,6 +256,27 @@
                         Grid.Column="1"
                         Width="400"
                         Margin="{StaticResource Margin4}" />
+
+                    <TextBlock
+                        x:Name="tbHttpHeaders"
+                        Grid.Row="3"
+                        Grid.Column="0"
+                        Margin="{StaticResource Margin4}"
+                        VerticalAlignment="Center"
+                        IsVisible="False"
+                        Text="{x:Static resx:ResUI.TbHttpOutboundHeaders}"
+                        ToolTip.Tip="{x:Static resx:ResUI.TipHttpOutboundHeaders}" />
+                    <views:JsonEditor
+                        x:Name="txtHttpHeadersJson"
+                        Grid.Row="3"
+                        Grid.Column="1"
+                        Width="400"
+                        MinHeight="100"
+                        Margin="{StaticResource Margin4}"
+                        HorizontalAlignment="Stretch"
+                        VerticalAlignment="Center"
+                        IsVisible="False"
+                        ToolTip.Tip="{x:Static resx:ResUI.TipHttpOutboundHeaders}" />
                 </Grid>
                 <Grid
                     x:Name="gridVLESS"