Add a python binding for libmongocrypt using "in-line", "ABI mode" CFFI. ABI mode does not require the user to have a C compiler during installation or runtime but it does require the user to install the libmongocrypt SO manually.

The benefits to ABI mode mode is that:

• the user does not need to install a C compiler during installation or runtime
• Simplifies build and release process because we can (likely?) build a universal wheel instead of a wheel for every platform X python version.
• the user can upgrade the libmongocrypt SO.

Cons:

• The user has to manually install libmongocrypt. This makes it more likely for the user to misconfigure the installation, eg installing an incompatible version of libmongocrypt.