Hi! Thank you for the great tool!

Could you please take a look at this PR?

Description

This PR adds support for OAUTH2 device flow:

• RFC: https://datatracker.ietf.org/doc/html/rfc8628
• Microsoft: https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-device-code

Device flow is particularly useful on headless systems, as it does not require usage of the redirect URL.

I successfully tested this PR with a free Microsoft account (outlook.com), and own application client_id.
The application (client_id) must be explicitly allowed to use the device flow in the Azure console.

Implementation

The following properties are introduced into the request propagated from OAUTH2 to the main app:

• need_response: A boolean specifying the flow implies a response from a user to be provided to the proxy in order to retrieve the tokens. It is false for device flow, and true for others.
• user_code: A (short) string, which should be provided to the user for authentication at the service provider. User should manually enter it at the provider's web page.

The following changes are done to the UI, both graphical and console:

• external auth: the user code is printed to the console together with the verification URI.
• local server auth: the user code is printed to the console together with the verification URI.
• icon: the user code is added to the title of the authorization window.

QA

I tested the proxy on Linux systems, in container and on desktop.
The following test cases are successful:

• external auth: the verification URI and user code are printed to the console.
• local server auth: the verification URI and user code are printed to the console.

Attention! On my systems I cannot make GUI work at all (even without my changes). The icon in the tray is always frozen, without notifications, and without context menu.
Therefore I cannot validate that the changes are correct for GUI interaction.