Allow multiple values for one key when adding web request parameters

Couldn't be me fixing this for FILE parameters
(ppy#6412) and then completely
failing to notice that `AddParameter()` in fact *OVERWRITES* the
previous value for a given key.

This failure is the root cause for
ppy/osu#32059. The difference between the
online version of the beatmap and the local version provided by the user
is that two `.osu` files changed filename inadvertently, because of
lazer escaping quote characters (which stable doesn't do).

In terms of patching, submission represents this as deleting the two
files from the online version of the package and then adding another
two. However, because `AddParameter()` would silently *ONLY KEEP THE
LAST FILENAME SPECIFIED* to delete, one of the `.osu` files would remain
in the patched package, and show up as a "tag mismatch" while it was
supposed to be completely gone.

---

## Breaking changes

`WebRequest.AddParameter()` multiple times with the same first parameter
(`name`) will no longer overwrite previous calls, which is to say, a
single parameter name is allowed to be associated with multiple values.

Author: bdach

osu.Framework/IO/Network/WebRequest.cs

@@ -109,12 +109,12 @@ public class WebRequest : IDisposable
         /// <summary>
         /// Query string parameters.
         /// </summary>
-        private readonly Dictionary<string, string> queryParameters = new Dictionary<string, string>();
+        private readonly List<(string key, string value)> queryParameters = new List<(string key, string value)>();
 
         /// <summary>
         /// Form parameters.
         /// </summary>
-        private readonly Dictionary<string, string> formParameters = new Dictionary<string, string>();
+        private readonly List<(string key, string value)> formParameters = new List<(string key, string value)>();
 
         /// <summary>
         /// FILE parameters.
@@ -304,7 +304,7 @@ private async Task internalPerform(CancellationToken cancellationToken = default
 
                     StringBuilder requestParameters = new StringBuilder();
                     foreach (var p in queryParameters)
-                        requestParameters.Append($@"{p.Key}={Uri.EscapeDataString(p.Value)}&");
+                        requestParameters.Append($"{p.key}={Uri.EscapeDataString(p.value)}&");
                     string requestString = requestParameters.ToString().TrimEnd('&');
                     url = string.IsNullOrEmpty(requestString) ? url : $"{url}?{requestString}";
 
@@ -345,7 +345,7 @@ private async Task internalPerform(CancellationToken cancellationToken = default
                             var formData = new MultipartFormDataContent(form_boundary);
 
                             foreach (var p in formParameters)
-                                formData.Add(new StringContent(p.Value), p.Key);
+                                formData.Add(new StringContent(p.value), p.key);
 
                             foreach (var p in files)
                             {
@@ -681,7 +681,7 @@ public void AddFile(string paramName, byte[] data, string filename = "blob")
 
         /// <summary>
         /// <para>
-        /// Add a new parameter to this request. Replaces any existing parameter with the same name.
+        /// Add a new parameter to this request.
         /// </para>
         /// <para>
         /// If this request's <see cref="Method"/> supports a request body (<c>POST, PUT, DELETE, PATCH</c>), a <see cref="RequestParameterType.Form"/> parameter will be added;
@@ -701,7 +701,7 @@ public void AddParameter(string name, string value)
             => AddParameter(name, value, supportsRequestBody(Method) ? RequestParameterType.Form : RequestParameterType.Query);
 
         /// <summary>
-        /// Add a new parameter to this request. Replaces any existing parameter with the same name.
+        /// Add a new parameter to this request.
         /// <see cref="RequestParameterType.Form"/> parameters may not be used in conjunction with <see cref="AddRaw(Stream)"/>.
         /// </summary>
         /// <remarks>
@@ -718,14 +718,14 @@ public void AddParameter(string name, string value, RequestParameterType type)
             switch (type)
             {
                 case RequestParameterType.Query:
-                    queryParameters[name] = value;
+                    queryParameters.Add((name, value));
                     break;
 
                 case RequestParameterType.Form:
                     if (!supportsRequestBody(Method))
                         throw new ArgumentException("Cannot add form parameter to a request type which has no body.", nameof(type));
 
-                    formParameters[name] = value;
+                    formParameters.Add((name, value));
                     break;
             }
         }