Formal Verification - Lean4 Integration

[ivan-randjelovic-cp]

This PR involves integration of Lean4 into xrpld. Please look at docs/formal-verification/README.md.

The README covers the Lean4 to C++ integration end to end: the motivation, terminology, an architecture overview, the build/integration flow (Conan toolchain -> CMake -> tests), how the Lean4 model is written, and how the FFI bridge lets C++ call it.

The PR adds an example Lean4 model (Number), properties/theorems proved about it, and an example C++ unit test that cross-validates the Lean4 model against the real C++.

I apologize for not using the usual PR description, as this is an out-of-ordinary type of a pull request.

[github-actions]

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[github-actions]

This PR has conflicts, please resolve them in order for the PR to be reviewed.