feat: Adds cancel invitation

* Added api request to cancel an organization invitation

* Added tests

---------

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

Octokit.Reactive/Clients/IObservableOrganizationMembersClient.cs

@@ -363,7 +363,19 @@ namespace Octokit.Reactive
         /// <param name="options">Options to change API behaviour</param>
         /// <returns></returns>
         IObservable<OrganizationMembershipInvitation> GetAllFailedInvitations(string org, ApiOptions options);
-        
+
+        /// <summary>
+        /// Cancel an organization invitation. In order to cancel an organization invitation, the authenticated user must be an organization owner.
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://docs.github.com/en/rest/orgs/members#cancel-an-organization-invitation">API Documentation</a>
+        /// for more information.
+        /// </remarks>
+        /// <param name="org">The login for the organization</param>
+        /// <param name="invitationId">The unique identifier of the invitation</param>
+        /// <returns></returns>
+        IObservable<Unit> CancelOrganizationInvitation(string org, int invitationId);
+
         /// <summary>
         /// Returns all <see cref="OrganizationMembership" />s for the current user.
         /// </summary>

Octokit.Reactive/Clients/ObservableOrganizationMembersClient.cs

@@ -507,6 +507,25 @@ namespace Octokit.Reactive
             return _connection.GetAndFlattenAllPages<OrganizationMembershipInvitation>(ApiUrls.OrganizationFailedInvitations(org), null, options);
         }
 
+        /// <summary>
+        /// Cancel an organization invitation. In order to cancel an organization invitation, the authenticated user must be an organization owner.
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://docs.github.com/en/rest/orgs/members#cancel-an-organization-invitation">API Documentation</a>
+        /// for more information.
+        /// </remarks>
+        /// <param name="org">The login for the organization</param>
+        /// <param name="invitationId">The unique identifier of the invitation</param>
+        /// <returns></returns>
+        [ManualRoute("DELETE", "/orgs/{org}/invitations/{invitation_id}")]
+        public IObservable<Unit> CancelOrganizationInvitation(string org, int invitationId)
+        {
+            Ensure.ArgumentNotNullOrEmptyString(org, nameof(org));
+            Ensure.ArgumentNotNullOrDefault(invitationId, nameof(invitationId));
+
+            return _client.CancelOrganizationInvitation(org, invitationId).ToObservable();
+        }
+
         /// <summary>
         /// Returns all <see cref="OrganizationMembership" />s for the current user.
         /// </summary>