Migrate project to use FetchContent and custom CMake files

[traversaro]

As the probability of the improvements in the CMake being merged upstream is really low, I think it would improve maintainability to strictly separate the raw files coming from the qpOASES tarball, and our own CMake files. In this way, updating to a new qpOASES should be simpler. One possibility is event to fetch the source code during the configuration phase using CMake's FetchContent module.
I think this projects would be a good example:

• https://github.com/robotology-dependencies/libxml2-cmake-buildsystem
• https://github.com/python-cmake-buildsystem/python-cmake-buildsystem

[traversaro]

As we have submitted upstream the changes that we need as described in #1, I think we hopefully can switch to use the upstream directly at some point in the future, so there is no need to setup this.