Rework VTK extensions

[Meakk]

Currently, we have implemented 4 different VTK extensions (Applicative, Core, Rendering, Readers).
These VTK extensions are private and not part of the SDK.
However, we foresee some cases where it can be useful to expose utils or even filters for the plugins.
A example could be having a filter to handle face-varying attributes for Alembic and USD formats.

Here's the different sub-tasks:

• Merge the VTK extensions to a single extension
• Use shared library instead of forcing a static library
• Export symbols for correct usage on Windows
• Install the library in the SDK and create a specific CMake target like f3d::vtkext

[mwestphal]

Still need to merge all private extensions into one

[Meakk]

Can we close it?