Add libcsp Rust integration layer with Linux I2C support

[OrenRotaru]

Summary

• Adds vendor/libcsp and vendor/libcsp-rust as git submodules, brings the CubeSat Space Protocol C library and its Rust FFI bindings.
• Introduces a new radsat-csp crate (kubos/libs/radsat-csp) that wraps libcsp behind an Rust API usable by device APIs and services.
• Adds a vendor patch and helper script (scripts/apply-vendor-patches.sh) to fix libcsp-rust raw-pointer slice expressions rejected by current Rust compilers under dangerous_implicit_autorefs.

radsat-csp crate features

• One-time initialization — initialize() backed by std::sync::Once; called automatically by RouterWorker and ReservedServiceWorker.
• RouterWorker — spawns the libcsp routing thread and keeps it alive for the process lifetime.
• ReservedServiceWorker — handles standard CSP reserved-port services (ping, etc.).
• CspClient — helpers for ping, send, and request/reply transaction with configurable timeout and priority.
• Linux I2C transport (--features i2c) — LinuxI2cCspInterface registers libcsp's C I2C interface and uses linux-embedded-hal::I2cdev for physical writes. Includes route_node_via_i2c_addr() for when a device's CSP node id differs from its 7-bit I2C address, and explicit read_frame_from() / inject_received_frame() helpers for master-driven receive polling.

Test plan

• cargo build -p radsat-csp builds cleanly after git submodule update --init --recursive && sh scripts/apply-vendor-patches.sh
• cargo build -p radsat-csp --features i2c builds with I2C support enabled
• cargo test -p radsat-csp passes the loopback smoke test (ping + echo transaction over the CSP loopback interface)
• cargo test -p radsat-csp --features i2c passes the I2C command/frame encoding tests