[ignition-math6] Add new port 🤖

[traversaro]

• What does your PR fix?

  • This PR adds a new port for the new major version of the ignition-math library.

• Which triplets are supported/not supported?

  • All tested triplet should be supported

• Does your PR follow the maintainer guide?

  • I hope.

As discussed in #7781, different major version of ignition robotics libraries (https://ignitionrobotics.org/) can be installed side by side, so each new major version is added as a new port.

In particular, ignition-math6 is a dependency for Gazebo 11 (http://gazebosim.org/blog/gazebo11) and for Ignition Robotics Citadel (that contains Ignition Gazebo 3, see https://www.openrobotics.org/blog/2019/12/11/ignition-citadel-released).

To fix the build on arm64-windows, this PR also bumps the ignition-cmake2 version to 2.2.0, that contains a fix for arm64-windows (https://osrf-migration.github.io/ignition-gh-pages/#!/ignitionrobotics/ign-cmake/pull-requests/168/page/1).

[traversaro]

The Linux CI is failing on the libqcow port with this error:

-- Note: libqcow only supports dynamic library linkage. Building dynamic library.
-- Downloading https://github.com/libyal/libqcow/releases/download/20191221/libqcow-alpha-20191221.tar.gz...
-- Downloading https://github.com/libyal/libqcow/releases/download/20191221/libqcow-alpha-20191221.tar.gz... Failed. Status: 22;"HTTP response code said error"
CMake Error at scripts/cmake/vcpkg_download_distfile.cmake:175 (message):
      
      Failed to download file.
      If you use a proxy, please set the HTTPS_PROXY and HTTP_PROXY environment
      variables to "https://user:password@your-proxy-ip-address:port/".
      Otherwise, please submit an issue at https://github.com/Microsoft/vcpkg/issues

Call Stack (most recent call first):
  ports/libqcow/portfile.cmake:8 (vcpkg_download_distfile)
  scripts/ports.cmake:90 (include)

That I guess it is both unrelated and a network flake.

[NancyLi1013]

LGTM now, thanks for this PR @traversaro.

[strega-nil]

Hey, this looks great! Thanks @traversaro