fix: change user-agent at runtime

[imLinguin]

This should address the login issue. In the meantime simply going through Heroic's Manage Accounts page will let users log in.
This PR fixes the regression when going to login through Stores tab

Closes #4477
Closes #4460
Closes #4433
Closes #4419
Closes #4289
Closes #4223
Closes #4430

Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[arielj]

Just curious, why is the at runtime part needed? I think it would be good to add a comment about that in the code so we don't break this accidentally in a refactor or something

[Etaash-mathamsetty]

Just curious, why is the at runtime part needed? I think it would be good to add a comment about that in the code so we don't break this accidentally in a refactor or something

Otherwise the user agent is only set once for the first URL and is never updated again

[imLinguin]

exactly what @Etaash-mathamsetty said, useragent provided to webview tag is a one-time property. Since we need different user agent for Epic's login and different for Epic's website, we need to be able to change that property at any point while webview is rendered.
Most issues I see in the issues I linked are related to users using Store tab, which as it is now, wouldn't change user-agent when redirected to Heroic's Epic login route

[Etaash-mathamsetty]

lgtm