re_web_viewer_server no longer needs tokio, split out sync code path

[Wumpf]

What

• Followup to Rewrite re_ws_comms to work without async & tokio runtime #6005
• another piece of Consider getting rid of tokio #5907

Removes tokio dependency from re_web_viewer_server:

• async WebViewerServer no longer uses tokio internally, instead use future_channel::oneshot for shutdown
• WebViewerServerHandle is now optional (but default enabled) and spawns a thread that internally uses the tiny pollster crate
• removed re_web_viewer_server's main file - it's not used by any test and may thus rot
  • this allows us to also remove the clap dependency from this crate

⚠️ Rerun serve/connect still needs a tokio runtime with this change ⚠️: Likely the last piece after this is re_sdk_comms which is very similar in nature to re_ws_comms.

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
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