Add various property validation in EmbedBuilder (#711)

* Add various property validation in EmbedBuilder

* Embed URI changes

Changes property types for any URLs in Embeds to System.URI.
Adding field name/value null/empty checks.

* including property names in argumentexceptions

* Adds overall embed length check

src/Discord.Net.Rest/API/Common/EmbedImage.cs

@@ -1,4 +1,5 @@
 #pragma warning disable CS1591
+using System;
 using Newtonsoft.Json;
 
 namespace Discord.API
@@ -6,9 +7,9 @@ namespace Discord.API
     internal class EmbedImage
     {
         [JsonProperty("url")]
-        public string Url { get; set; }
+        public Uri Url { get; set; }
         [JsonProperty("proxy_url")]
-        public string ProxyUrl { get; set; }
+        public Uri ProxyUrl { get; set; }
         [JsonProperty("height")]
         public Optional<int> Height { get; set; }
         [JsonProperty("width")]