Support Python 3.12

[s0l0ist]

Description

This PR introduces support for Python 3.12. In earlier versions, the Python bindings did not properly acquire the Global Interpreter Lock (GIL) when returning Python objects, particularly around the GetPrivateKeyBytes pybind11 definitions. Since we are returning a py::bytes object, we should have been acquiring the GIL, but instead, we were incorrectly releasing it.

In previous versions, this behavior could lead to undefined behavior or potential crashes when Python objects were accessed without the GIL being held (although in practice this was not observed). The refactor ensures thread safety by retaining scoped GIL release calls while now correctly acquiring the lock when exceptions are raised back to Python. This improves consistency and correctness when interacting with Python objects.

Also updates the rustc version to the latest (v1.81.0) and PCJ latest changes (removed unused bazel configuration for GRPC)

Affected Dependencies

N/A

How has this been tested?

• A new test_3_12 bazel test which is now included in CI workflow

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests