feat(native-certgen): scaffold Rust crate with foundation types and keygen

Cargo.toml with 16 dependencies per build spec, error types with
From impls for all upstream error types, CertGenParams mapping the
full JNI config contract, EC/RSA key generation via ring and rsa crates.

Compiles clean for aarch64-linux-android via cargo-ndk.

native-certgen/src/lib.rs

@@ -0,0 +1,7 @@
+mod error;
+mod types;
+mod keygen;
+// mod attestation;  // Phase 2
+// mod keybox;       // Phase 0-1 Task 2
+// mod certbuilder;  // Phase 3
+// mod logging;      // Phase 3