add active lock reason to PR (#2543)

* add active lock reason to PR

* update docs

* refactor: extract lock and unlock from IIssuesClient
create ILockUnlockClient so both IIssuesClient and IPullRequestClient can
access lock and unlock methods.

* refactor LockUnlock for reactive clients

* Update doc to include lock unlock sample code

* Use Assert.Null to check null value in test

Co-authored-by: notauserx <notauserx@users.noreply.github.com>

Octokit/Clients/IPullRequestsClient.cs

@@ -28,6 +28,11 @@ namespace Octokit
         /// </summary>
         IPullRequestReviewRequestsClient ReviewRequest { get; }
 
+        /// <summary>
+        /// Client for locking/unlocking a coversation on a pull request
+        /// </summary>
+        ILockUnlockClient LockUnlock { get; }
+
         /// <summary>
         /// Get a pull request by number.
         /// </summary>