Support external dependencies

[uilianries]

Using only third party folder keeps this project stuck over the same version always.

This PR includes the CMake option to be able to use external dependencies (magic_enum and visit_struct).

Some extra changes:

• Moved the third party folder location, to reflect its role, an external project.
• Updated the local copy of magic_enum to the latest version (0.3.5)
• Add STRUCTOPT_EXTERNAL_DEPS to use magic_enum and visit_struct from the system
• Removed Conanfile recipe, because is now part of official Conan Center: https://conan.io/center/recipes/structopt

[perseoGI]

Hi @p-ranav, did you have the chance to dig into this PR?

[Ahajha]

@uilianries A PR to update the vendored dependencies was just merged, you can probably simplify this PR

[uilianries]

@Ahajha Thank you for pinging!