feat(core): expose user_agent to window config

[emhagman]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Requested here: #3904

• Allow setting userAgent in the config file for any window
• Added example under examples/user-agent

{
    "windows": [
      {
        "title": "Welcome to Tauri!",
        "width": 800,
        "height": 600,
        "resizable": true,
        "fullscreen": false,
        "userAgent": "Tauri 1.0 (Custom Agent)"
      }
    ]
}

Screenshots

Before	After

[lucasfernog]

Nice contribution, thank you so much!

[emhagman]

@lucasfernog Thanks! And thanks for the tweaks, I'll be sure to follow that pattern next time 👍🏼