[serf] new port (build using scons)

[rinrab]

In this version on the port, we are going to use the official build system of the library, Scons. There is already a helper for getting it from the internet, but it wasn't actually working, and should be it with Python should be updated.

Included changes:

• Update scons version to the latest.
• Using Python3 instead of Python2 for scons.
• Add the port for Serf library

TODO List:

• Insure that all Azure Pipelines cases are passing.
• Check that Subversion can be compiled correctly against Serf from this port.
• Add the port version.
• Test the port against different configurations.
• Add pretty logging.

[rinrab]

It worked on my Linux VM, but fails on Azure Pipelines, due to missing scons on the system. I have a question: how to make it installed there? I've already specified apt_package_name and brew_package_name variables in the vcpkg_find_acquire_program(SCONS).cmake script and it suggests to install this tool.