[FEAT]: Add support for Public Keys API (#2945)

* Add support for /mets/public_keys/<keyType>

* "files.insertFinalNewline": false

* revert and make setttings.json change csharp only

* formatting

* remove final new line

---------

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>

Octokit.Tests/Reactive/ObservablePublicKeysClientTests.cs

@@ -0,0 +1,33 @@
+using System;
+using NSubstitute;
+using Octokit.Reactive;
+using Xunit;
+
+namespace Octokit.Tests.Reactive
+{
+    public class ObservablePublicKeysClientTests
+    {
+        public class TheGetMethod
+        {
+            [Fact]
+            public void CallsIntoClient()
+            {
+                var gitHubClient = Substitute.For<IGitHubClient>();
+                var client = new ObservablePublicKeysClient(gitHubClient);
+
+                client.Get(PublicKeyType.SecretScanning);
+
+                gitHubClient.Meta.PublicKeys.Received(1).Get(PublicKeyType.SecretScanning);
+            }
+        }
+
+        public class TheCtor
+        {
+            [Fact]
+            public void EnsuresNonNullArguments()
+            {
+                Assert.Throws<ArgumentNullException>(() => new ObservablePublicKeysClient((IGitHubClient)null));
+            }
+        }
+    }
+}