Make Qt WebSockets examples self-contained

The sslechoclient example referenced ../sslechoserver/localhost.cert via
its CMakeLists.txt and .pro file, violating QUIP-13 which requires that
examples not reference source or asset files outside their own directory.
This breaks "copy example out of tree" in Qt Creator and VS Code.

The example now ships its own copy of localhost.cert and references it
locally. The sslechoserver example keeps its own copy as the owner of
the original.

Task-number: QTBUG-147517
Pick-to: 6.11 6.12
Change-Id: Ic16da57cc33f5ba144ee10419a72d667caf0946c
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>