Add external action to install Qt into multiplatform CI actions

[JesusPoderoso]

This PR Includes a new multiplatform action that installs Qt in three platforms: linux, windows & mac.
All inputs from the external action have been included as not required without default value, with the following exceptions:

• version: default Qt version has been included. In this case, it is the same as the default version of the external action (5.15.2, last Qt5 LTS), as it is the Qt version used for developing eProsima Qt products.
• dir: set as required to force eProsima CI user to set a installation directory different from ../ (external action default dir). The desirable input dir would be ${{ github.workspace }}/qt_installation/, but it cannot be set as default.
• host and arch: they are not available input values, as their values are taken from the runner OS:

-	Linux	Windows	Mac
host	linux	windows	mac
arch	gcc_64	win64_msvc2019_64	clang_64

Update: to make it work, all the input default values must be set too

[rsanchez15]

Closed in favour of #119