Add X-Forwarded-For header name

tldr; add header name `X-Forwarded-For` as constant in `HeaderNames` static class.

Author: DenisBalan

src/Http/Headers/src/HeaderNames.cs

@@ -283,6 +283,9 @@ public static class HeaderNames
         /// <summary>Gets the <c>X-Frame-Options</c> HTTP header name.</summary>
         public static readonly string XFrameOptions = "X-Frame-Options";
 
+        /// <summary>Gets the <c>X-Forwarded-For</c> HTTP header name.</summary>
+        public static readonly string XForwardedFor = "X-Forwarded-For";
+
         /// <summary>Gets the <c>X-Requested-With</c> HTTP header name.</summary>
         public static readonly string XRequestedWith = "X-Requested-With";
 

src/Http/Headers/src/PublicAPI.Unshipped.txt

@@ -7,3 +7,4 @@ static readonly Microsoft.Net.Http.Headers.HeaderNames.Link -> string!
 static readonly Microsoft.Net.Http.Headers.HeaderNames.ProxyConnection -> string!
 static readonly Microsoft.Net.Http.Headers.HeaderNames.XContentTypeOptions -> string!
 static readonly Microsoft.Net.Http.Headers.HeaderNames.XXssProtection -> string!
+static readonly Microsoft.Net.Http.Headers.HeaderNames.XForwardedFor -> string!