(binbrk)feature: Support filtering audit log entries on user, action type, and before entry id (#1377)

* Support filtering audit log entries on user, type, and before id

Adds support for filtering audit log entires with GetAuditLogsAsync. Adds the ability to specify a userId and ActionType to filter. Exposes the beforeId filter which was already implemented, yet unused (even when requesting > 100 entries)?

Was thinking that this could expose overloads of GetAuditLogAsync that accepts a IUser and IAuditLogEntry, but dealing with all the combinations of these types may be excessive.

* use only stringbuilder for args instead of string interpolation

src/Discord.Net.Rest/API/Rest/GetAuditLogsParams.cs

@@ -1,8 +1,10 @@
-namespace Discord.API.Rest
+namespace Discord.API.Rest
 {
     class GetAuditLogsParams
     {
         public Optional<int> Limit { get; set; }
         public Optional<ulong> BeforeEntryId { get; set; }
+        public Optional<ulong> UserId { get; set; }
+        public Optional<int> ActionType { get; set; }
     }
 }