Ability to set request timeout for the DefaultRestClient

[IncontinentCell]

So temporarily I have really slow internet. My bot downloads images from attachments and sends them in a separate message. HttpClient inside of DefaultRestClient would constantly take too long and timeout causing it to not work. I looked into how all of this works underneath and I managed to modify the timeout using this ugly code using reflection.

config.RestClientProvider = url =>
{
    var rest = DefaultRestClientProvider.Instance(url);
    var type = rest.GetType();
    
    FieldInfo fi = type.GetField("_client", BindingFlags.NonPublic | BindingFlags.Instance);
    var field = (HttpClient)fi.GetValue(rest);
    field.Timeout = TimeSpan.FromMinutes(15);
    return rest;
};

Would be nice to be able to modify this timeout without doing all of this.

Also another thing I just noticed. Even though this made the message send I now get the following error in the logs:

System.Collections.Generic.KeyNotFoundException: The given key 'POST channels/1145108368084713602/messages' was not present in the dictionary.
   at System.Collections.Concurrent.ConcurrentDictionary`2.ThrowKeyNotFoundException(TKey key)
   at System.Collections.Concurrent.ConcurrentDictionary`2.get_Item(TKey key)
   at Discord.Net.Queue.RequestQueue.UpdateBucketHash(BucketId id, String discordHash)
   at Discord.Net.Queue.RequestBucket.UpdateRateLimit(Int32 id, IRequest request, RateLimitInfo info, Boolean is429, Boolean redirected, Stream body)
   at Discord.Net.Queue.RequestBucket.SendAsync(RestRequest request)
   at Discord.Net.Queue.RequestQueue.SendAsync(RestRequest request)
   at Discord.API.DiscordRestApiClient.SendInternalAsync(String method, String endpoint, RestRequest request)
   at Discord.API.DiscordRestApiClient.SendMultipartAsync[TResponse](String method, String endpoint, IReadOnlyDictionary`2 multipartArgs, BucketId bucketId, ClientBucketType clientBucket, RequestOptions options)
   at Discord.API.DiscordRestApiClient.UploadFileAsync(UInt64 channelId, UploadFileParams args, RequestOptions options)
   at Discord.Rest.ChannelHelper.SendFilesAsync(IMessageChannel channel, BaseDiscordClient client, IEnumerable`1 attachments, String text, Boolean isTTS, Embed embed, AllowedMentions allowedMentions, MessageReference messageReference, MessageComponent components, IS
ticker[] stickers, RequestOptions options, Embed[] embeds, MessageFlags flags)

I imagine that's probably because rate limit didn't expect the sending to take this long haha.