Don't fetch deps during build; use submodules

[rickwebiii]

When building Rust docs for our seal_fhe Rust wrapper crate, the docs fail to compile because docs.rs builds run in a Docker container that doesn't allow network access during compilation. This means third-party dependencies fail to download and the cmake bails.

This change migrates the third party dependencies to be submodules, avoiding the need to fetch them during the build. Rust will blindly package these as source files, so everyone should be happy.

I tested this by building SEAL with every feature enable and compiling tests and benchmarks.

[rickwebiii]

I should probably add a note in the README to run

git submodule init
git submodule update

after cloning your enlistment.