Application password: Fetch site username when authenticated with WPCom

[itsmeichigo]

Part of WOOMOB-1123

Description

This PR updates the default application password to fetch the site username to sign the password with when user is authenticated with WPCom token. Changes include:

• Triggers the request to GET wp/v2/users/me with edit context to get the site username.
• Moves User model, mapper, and remote from Networking to NetworkingCore module to be reused in DefaultApplicationPasswordUseCase.

Testing steps

The application password generation and deletion will be added in a subsequent PR so just unit tests passing is sufficient.

Testing information

Added unit tests for fetching username in application password use case.

Screenshots

No UI changes.

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16037-e5890fc
	Version	23.1
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	e5890fc
	Installation URL	19n57dng9t788

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[RafaelKayumov]

Code-wise the wporg username preloading looks good.