Add support for SAML based authentication when using OpenConnect VPN

This change utilizes QtWebEngine to provide an integrated browser whose data can
be forwarded to libopenconnect for handling the SAML authentication process.
This change also refactors the external browser SAML protocol flow to follow
conventions of the openconnect implementation in plasma-nm.

BUG: 448153
Signed-off-by: Rahul Rameshbabu <sergeantsagara@protonmail.com>