add test cases for tidbcloudy (#28)

- Support specifying timezone in `timestamp_to_string()` and `get_current_year_month()`
- Implement a mock server using Flask to facilitate testing
- Expand test coverage for the following API endpoints:

	- List the cloud providers, regions and available specifications
	- List all accessible projects
	- Create a project
	- List all clusters in a project
	- Get a cluster by ID
	- Create a cluster
	- Modify (pause or resume) a cluster
	- Delete a cluster
	- List AWS Customer-Managed Encryption Keys for a project
	- Configure AWS Customer-Managed Encryption Keys for a project

- Enhance testing for certain internal features:
    - the `to_object` and `from_object` functions of `TiDBCloudyBase`
    - `util.timestamp.py`
    - the `CreateClusterConfig` and `UpdateClusterConfig` in `tidbcloudy.specification`
    
- Fix the incorrect subclass and type hint of `BillingMonthSummaryByService`

README.md

@@ -71,6 +71,23 @@ You can use this SDK to access [TiDB Cloud](https://tidbcloud.com) and manage yo
   </tr>
 </thead>
 <tbody>
+  <tr>
+    <td><a href="https://github.com/Oreoxmt/tidbcloudy/releases/tag/v1.1.0" target="_blank" rel="noopener noreferrer">1.1.0</a></td>
+    <td>✅</td>
+    <td>❌</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>❌</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+    <td>✅</td>
+  </tr>
   <tr>
     <td><a href="https://github.com/Oreoxmt/tidbcloudy/releases/tag/v1.0.10" target="_blank" rel="noopener noreferrer">1.0.10</a></td>
     <td>✅</td>