[REST API] Add support for WordPress.com account creation as part of Jetpack Setup

[hichamboushaba]

Currently, in the Jetpack Setup flow, we don't support creating new accounts, which is counter-intuitive, as users who are installing Jetpack for the first time will most likely need to create a new account.

Using tracks, we can confirm this: currently, the login step is one of the biggest drops in the Jetpack Setup, as only 22% are able to finish it. From the failures, we can see that UNKOWN_USER is responsible for the biggest part of the cases.

Relevant internal discussion p1710847254725509-slack-C03L1NF1EA3

iOS GH issue: woocommerce/woocommerce-ios#12310

### Tasks
- [ ] https://github.com/woocommerce/woocommerce-android/issues/12918
- [ ] https://github.com/woocommerce/woocommerce-android/issues/12921
- [ ] https://github.com/woocommerce/woocommerce-android/issues/12944

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.