This commit adds a `TomcatWebSocketReactiveWebServerCustomizer`
that customizes the Tomcat context to accept WebSockets connections.
Since reactive servers don't use the JSR 356 for that support,
only Tomcat customization is required for now.

This commit also reorders the server auto-configuration
so that undertow has a chance to be auto-configured before
reactor-netty, which should be a popular dependency thanks to
its HTTP client library.

The existing WebSocket infrastructure for Serlvet based containers
has been moved to a dedicated package and renamed accordingly.

Fixes gh-9113