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.Tests/CoreConfig/CoreConfigTestFactory.cs

@@ -130,6 +130,25 @@ internal static class CoreConfigTestFactory
         };
     }
 
+    public static ProfileItem CreateHttpNode(ECoreType coreType, string indexId = "node-http-1",
+        string remarks = "demo-http")
+    {
+        return new ProfileItem
+        {
+            IndexId = indexId,
+            ConfigType = EConfigType.HTTP,
+            CoreType = coreType,
+            Remarks = remarks,
+            Address = "proxy.example.com",
+            Port = 8080,
+            Password = "pass",
+            Username = "user",
+            Network = nameof(ETransport.raw),
+            StreamSecurity = string.Empty,
+            Subid = string.Empty,
+        };
+    }
+
     public static ProfileItem CreatePolicyGroupNode(ECoreType coreType, string indexId, string remarks,
         IEnumerable<string> childIndexIds)
     {