examples Reverse ETL to upload my timesheet to Google Sheets (#1032)

* added Timewarrior timesheet ETL notebook
---------

Co-authored-by: zilto <tjean@DESKTOP-V6JDCS2>

examples/reverse_etl/README.md

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+# Reverse ETL for your timesheets
+
+This example shows how to create a timesheet from time tracking data generated by the command line tool [Timewarrior](https://timewarrior.net/) and upload it to Google Sheets.
+
+![](dag.png)
+
+## Content
+1. `notebook.ipynb` explains the code step-by-step and allows you to interact and modify the dataflow.
+2. `upload_timesheet.py` contains the same dataflow but as a script. It can be used as a [Timewarrior extension](https://timewarrior.net/docs/api/) directly, see below.
+
+## Set up
+1. Create a virtual environment and activate it
+    ```console
+    python -m venv venv && . venv/bin/active
+    ```
+
+2. Install requirements
+    ```console
+    pip install -r requirements.txt
+    ```
+
+## Timewarrior extension
+Adding a [Timewarrior extension](https://timewarrior.net/docs/api/) means you can trigger arbitrary scripts with `timew report SCRIPT_FILE_NAME`. Even better, you can skip the `report` command and only type the `SCRIPT_FILE_NAME` partially if it's unambiguous. For example, you can trigger `upload_timesheet.py` with `timew upload`. See the (notebook)[./notebook.ipynb] for further context and explanations.
+
+To enable this, follow these steps:
+1. Identify your Timewarrior installation directory (we'll assume the default: `~/.timewarrior`)
+2. Copy `upload_timesheet.py` from this example to `~/.timewarrior/extensions`
+3. Create a `credentials.json` with credentials for the Google Sheets API ([see step-by-step guide](https://hackernoon.com/how-to-use-the-google-sheets-api-with-python)) under `~/.timewarrior/extensions`. It should look like this:
+
+    ```json
+    credentials = {
+        "type": "service_account",
+        "project_id": "...",
+        "private_key_id": "...",
+        "private_key": "-----BEGIN PRIVATE KEY----- ...",
+        "client_email": "... .iam.gserviceaccount.com",
+        "client_id": "...",
+        "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+        "token_uri": "https://oauth2.googleapis.com/token",
+        "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+        "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/ ...",
+        "universe_domain": "googleapis.com"
+    }
+    ```
+4. Enable execution permissions with `chmod +x upload_timesheet.py`
+5. Verify the extension is enabled with `timew extensions`. You should see
+
+    ```console
+    Extensions located in:
+        ~/.timewarrior/extensions
+
+    Extension        Status
+    credentials.json No executable
+    upload_timesheet.py Active
+    ```
+6. Create a new Google Sheets spreadsheet and get its `spreadsheet_id` from the Google Sheets URL `https://docs.google.com/spreadsheets/d/$SPREADSHEET_ID/edit?gid=0#gid=0`
+7. Use the `Share` button to give access to the Google API service account that you created at Step 3
+8. Add the `spreadsheet_id` as a Timewarrior config point via `timew config spreadsheet_id $SPREADSHEET_ID`
+9. Use the command `timew report upload_timesheet.py` to test it out! You should be able to shorten it to `timew upload`
+
+> [!NOTE]
+> If you run `python upload_timesheet.py` will hang because it will try to read values from `sys.stdin` which should be provided by the Timewarrior CLI. That's why you need to run it via `timew report upload_timesheet.py`