Refactor for ref model to add external libraries as FetchContent modules

- Add external libraries like doctest, eigen, and json as FetchContent
modules instead of git submodules like they currently are.
- With this change, there is no need for the git submodule to be in
reference_model/thirdparty directory anymore, as their source code can
be access in build/_deps/<library-name>-src once ref model is built.

Signed-off-by: Justin Ngo <justin.ngo@arm.com>
Change-Id: I8796445d7abbbba2870975d71d22b5c31081c62f