[WebServerPlugin] Warn user about missing `--enable-web-server` flag when a web server plugin is used

[abhinavsingh]

Is your feature request related to a problem? Please describe.
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Describe the solution you'd like
Warn (or abort?) if user has loaded a http web server plugin but haven't specified --enable-web-server flag.

Describe alternatives you've considered
Alternately, --enable-web-server flag can be turned on implicitly if a web server plugin is detected.