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.Rest/Net/RateLimitInfo.cs

@@ -21,7 +21,7 @@ namespace Discord.Net
             Remaining = headers.TryGetValue("X-RateLimit-Remaining", out temp) && 
                 int.TryParse(temp, out var remaining) ? remaining : (int?)null;
             Reset = headers.TryGetValue("X-RateLimit-Reset", out temp) && 
-                int.TryParse(temp, out var reset) ? DateTimeOffset.FromUnixTimeSeconds(reset) : (DateTimeOffset?)null;
+                float.TryParse(temp, out var reset) ? DateTimeOffset.FromUnixTimeMilliseconds((long)(reset * 1000)) : (DateTimeOffset?)null;
             RetryAfter = headers.TryGetValue("Retry-After", out temp) &&
                 int.TryParse(temp, out var retryAfter) ? retryAfter : (int?)null;
             Lag = headers.TryGetValue("Date", out temp) &&