feat: Initiated pybind11 Configuration

[EmperorYP7]

Fixes #84 partially

Signed-off-by: Yash Pandey (YP) yash.btech.cs19@iiitranchi.ac.in

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Description

This code initiated the configuration of pybind11 into the project through CMake.

• I configured it so that it finds if pybind11 is installed locally. If it's not, the build system fetches version 2.7.0 from GitHub into the build/_deps directory and makes it available to our targets.
• Added a target named pycasbin which compiles to a dynamic library i.e. a python module which can be imported in Python code.
• Added an option CASBIN_BUILD_BINDINGS to specify whether to build language bindings, and CASBIN_BUILD_PYTHON_BINDINGS to toggle builds specific to python bindings.
• When disabled, CASBIN_BUILD_BINDINGS and CASBIN_BUILD_PYTHON_BINDINGS won't fetch anything from GitHub adding modularity to our build system.
• Added a mock binding for

  int sum(int a, int b);
  [](int i, int j){ return i - j; }

  referring to pybind11's docs.
• Removed the makefile since it is now auto-generated through CMake.

Noteworthy

• Although the build system works well and needs minimal configuration from the user's standpoint, I encountered many errors related to C++ standard used when trying to link pycasbin with the casbin target.

• Vital template definitions like std::variant were undefined. This is becuase pycasbin is built with C++11 standard while casbin is compiled with C++17 standard.

• I referred (no copy-pasting) to the build system config for pybind11 from AcademySoftwareFoundation/OpenColorIO for this as I think it was suitable for our use case.

[hsluoyz]

@xcaptain @divy9881

[EmperorYP7]

@xcaptain @divy9881 @hsluoyz
I think we should merge this to avoid merge conflicts in the subsequent PRs.

[github-actions]

🎉 This PR is included in version 1.28.0 🎉

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