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

---------

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

v2rayN/ServiceLib/Models/CoreConfigs/V2rayConfig.cs

@@ -146,6 +146,16 @@ public class Outboundsettings4Ray
 
     public int? port { get; set; }
 
+    public string? user { get; set; }
+
+    public string? pass { get; set; }
+
+    public int? level { get; set; }
+
+    public string? email { get; set; }
+
+    public object? headers { get; set; }
+
     public List<WireguardPeer4Ray>? peers { get; set; }
 
     public bool? noKernelTun { get; set; }

v2rayN/ServiceLib/Models/Entities/ProtocolExtraItem.cs

@@ -5,6 +5,9 @@ public record ProtocolExtraItem
     public bool? Uot { get; init; }
     public string? CongestionControl { get; init; }
 
+    // http outbound
+    public string? HttpHeaders { get; init; }
+
     // vmess
     public string? AlterId { get; init; }
     public string? VmessSecurity { get; init; }