[bug] WebviewBuilder options on the JS api other than the file drop handler gets discarded

[Hyphrio]

Describe the bug

It seems like that looking at the associated Rust code, defined on the tauri::window::WindowBuilder::from_config, Webview options other than the file drop handler, gets discarded.

The code:

pub fn from_config<M: Manager<R>>(manager: &'a M, config: WindowConfig) -> Self {
    let runtime = manager.runtime();
    let app_handle = manager.app_handle();
    let url = config.url.clone();
    let file_drop_enabled = config.file_drop_enabled;
    let mut builder = Self {
      manager: manager.manager().clone(),
      runtime,
      app_handle,
      label: config.label.clone(),
      window_builder: <R::Dispatcher as Dispatch<EventLoopMessage>>::WindowBuilder::with_config(
        config,
      ),
      webview_attributes: WebviewAttributes::new(url),
      web_resource_request_handler: None,
      navigation_handler: None,
    };

    if !file_drop_enabled {
      builder = builder.disable_file_drop_handler();
    }

    builder
  }

Looking at webview_attributes, it looks like it never sees the config, and just resorts to the default one.

Reproduction

First, use WebviewBuilder to create a window, passing anything other than url and fileDropEnabled.
Minimal reproduction example:

async function createWindow() {
    new WebviewBuilder("example", {url: "https://www.whatsmyua.info", userAgent: "tauri/1.0"});
}

Expected behavior

In this User-Agent example, the user agent should be tauri/1.0.

However, what appears is the default user agent of the browsers used in each platform.

Any other configs should be affected as I discovered it while doing #6767.

Platform and versions

While my development platform is as follows:
[✔] Environment
    - OS: Windows 10.0.22621 X64
    ✔ WebView2: 112.0.1722.58
    ✔ MSVC: Visual Studio Build Tools 2022
    ✔ rustc: 1.67.1 (d5a82bbd2 2023-02-07)
    ✔ Cargo: 1.67.1 (8ecd4f20a 2023-01-10)
    ✔ rustup: 1.25.2 (17db695f1 2023-02-01)
    ✔ Rust toolchain: stable-x86_64-pc-windows-msvc (default)
    - node: 18.15.0
    - pnpm: 8.1.1
    - yarn: 1.22.19
    - npm: 9.5.0

[-] Packages
    - tauri [RUST]: git+https://github.com/Hyphrio/tauri?branch=incognito#e0866b5920fed0e91a5a308c6d152093b57006c2 (2.0.0-alpha.8)
    - tauri-build [RUST]: git+https://github.com/Hyphrio/tauri?branch=incognito#e0866b5920fed0e91a5a308c6d152093b57006c2 (2.0.0-alpha.4)
    - wry [RUST]: 0.27.3
    - tao [RUST]: 0.18.3
    - @tauri-apps/api [NPM]: not installed!
    - @tauri-apps/cli [NPM]: 2.0.0-alpha.8

[-] App
    - build-type: bundle
    - CSP: unset
    - distDir: ../dist
    - devPath: http://localhost:7000/
    - framework: React
    - bundler: Vite

All platforms should be affected including Tauri 1.0 as that also uses the same code.

Stack trace

No response

Additional context

Building the Webview using WindowBuilder::from_config would be also affected by this issue, however most users would probably not be affected as they use WindowBuilder::new instead.

This code should probably be refactored to fix this issue.