feature: Add support for setting X-RateLimit-Precision (#1354)

* support X-RateLimit-Reset sending integer or float values

This changes the way that the X-RateLimit-Request header is parsed, so that it will work with both integer seconds and float values with seconds and milliseconds

* Add RateLimitPrecision enum, set X-RateLimit-Precision

Adds the RateLimitPrecision enum, with Second and Millisecond values. (Do we want to use an extension method to convert it into a string, or is ToString().ToLower() fine?)
Adds RateLimitPrecision as a parameter to DiscordRestApiClient, and to DiscordConfig so that it can set the X-RateLimit-Precision header.

src/Discord.Net.Core/RateLimitPrecision.cs

@@ -0,0 +1,18 @@
+namespace Discord
+{
+    /// <summary>
+    ///     Specifies the level of precision to request in the rate limit
+    ///     response header.
+    /// </summary>
+    public enum RateLimitPrecision
+    {
+        /// <summary>
+        ///     Specifies precision rounded up to the nearest whole second
+        /// </summary>
+        Second,
+        /// <summary>
+        ///     Specifies precision rounded to the nearest millisecond.
+        /// </summary>
+        Millisecond
+    }
+}