Allow passing a "source type" when creating Woo Orders.

[hichamboushaba]

Part of woocommerce/woocommerce-android#10990

This PR updates the Woo Order store by allowing passing a custom "source type" to attribute the order to (check woocommerce/woocommerce-android#10960 for more details).

Instead of hardcoding the value of the mobile app in FluxC, I opted for making a configurable value, where each client can use their own, even though for now WCAndroid is the only client.

Testing

1. Open the example app and sign in.
2. Click on Woo, then Orders.
3. Select a site.
4. Click on Create an order.
5. Enter a product ID.
6. Skip the other steps.
7. When the order is created, check it in wp-admin, and confirm it has a source type set to: "fluxc-example-app" (the Origin will be unknown, as this source type value is not mapped by Core)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[0nko]

The test works as expected 👍