Split extension methods into separate classes (#161)

This PR splits the extension methods contained in `Extensions.cs` into
separate classes in the `Octobot.Extensions` namespace. This was done
for multiple reasons:
1) The `Extensions.cs` violates SRP (Single Responsibility Principle) -
it takes upon itself every extension method for many types
2) Having a separate class for each extended type is a standard practice
- take a look at
[Remora.Discord](https://github.com/Remora/Remora.Discord/tree/main/Backend/Remora.Discord.Rest/Extensions)
or [osu!](https://github.com/ppy/osu/tree/master/osu.Game/Extensions)
3) Having all extension methods in one file makes it hard to find the
method you want

src/Extensions/GuildScheduledEventExtensions.cs

@@ -0,0 +1,28 @@
+using Remora.Discord.API.Abstractions.Objects;
+using Remora.Rest.Core;
+using Remora.Results;
+
+namespace Octobot.Extensions;
+
+public static class GuildScheduledEventExtensions
+{
+    public static Result TryGetExternalEventData(this IGuildScheduledEvent scheduledEvent, out DateTimeOffset endTime,
+        out string? location)
+    {
+        endTime = default;
+        location = default;
+        if (!scheduledEvent.EntityMetadata.AsOptional().IsDefined(out var metadata))
+        {
+            return new ArgumentNullError(nameof(scheduledEvent.EntityMetadata));
+        }
+
+        if (!metadata.Location.IsDefined(out location))
+        {
+            return new ArgumentNullError(nameof(metadata.Location));
+        }
+
+        return scheduledEvent.ScheduledEndTime.AsOptional().IsDefined(out endTime)
+            ? Result.FromSuccess()
+            : new ArgumentNullError(nameof(scheduledEvent.ScheduledEndTime));
+    }
+}