Replace host_web_viewer method with WebViewerConfig::host_web_viewer

[Wumpf]

What

This makes it easier to host Rerun webviewer.

Usage example

    rerun::web_viewer::WebViewerConfig {
        source_url: Some(
            "https://app.rerun.io/version/0.18.1/examples/arkit_scenes.rrd".to_owned(),
        ),
        force_wgpu_backend: Some("webgl".to_owned()),
        ..Default::default()
    }
    .host_web_viewer()?
    .block();

This is a followup of this comment #7537 (comment)
Opted to not implement a builder as well since I felt it's not worth the hassle here, above snippet looks perfectly fine without a builder.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.

[emilk]

LGTM if tested 👍

[Wumpf]

Did some via pixi rerun-web, but yeah let's see if we can do some more!