[SCons] Add GDExtension method to scons.

[Faless]

Called like:

env.GDExtension("my_library_name", sources)

Optional parameters:

• entry_symbol = "" ({name}_library_init if empty)
• output_dir = "" (output extension folder, {name} if empty)
• library_dir = "lib" (library subdir, relative to the output_dir)

Builds:

{output_dir}/
  {name}.gdextension
  {library_dir}/
    lib{name}.{platform-arch-library-suffix}

Draft:

• Optional override for for .gdextension file (copy instead of generate).
• Extension library auto detect feature in Godot (cc @groud )
• Handle macos framework and their custom structure/Info.plist (auto-generate with optional override? Have it handled by godot platform exporter? cc @bruvzg )