refactor(rust_crypto): move RSA backend to rsa 0.10.0-rc.17

[EffortlessSteven]

Addresses #495.

I kept this to the rust_crypto RSA seam.

Scope

• rust_crypto backend only
• no public API changes
• no aws_lc_rs changes
• no crate-wide signature 3 migration
• no crate-wide sha2 0.11 migration

Dependency note

• This uses the currently published crates.io 0.10 line for RSA, which is rsa 0.10.0-rc.17.

Review map

• Cargo.toml
• src/crypto/rust_crypto/rsa.rs
• src/crypto/rust_crypto/ecdsa.rs
• src/crypto/rust_crypto/mod.rs

What changed

• replace wrapper-based RSA signing with explicit prehash + low-level key operations
• keep verification on RsaPublicKey::verify(...)
• move RSA modulus/exponent handling to BoxedUint
• decouple ECDSA from rsa::pkcs8
• preserve minimal big-endian n/e extraction for DecodingKey::from_rsa_components and JWK generation
• add a wasm-target-only getrandom 0.4 alias so the existing wasm-pack commands keep working

Existing coverage already exercising the touched paths

• PEM PKCS#1 and PKCS#8 RSA round-trips
• DER round-trips
• modulus/exponent decode via DecodingKey::from_rsa_components
• JWK decode
• JWK generation from an encoding key

Validated locally

• cargo fmt --check
• cargo clippy --all-targets --features rust_crypto -- -D warnings
• cargo clippy --all-targets --features aws_lc_rs -- -D warnings
• cargo test --features rust_crypto
• cargo test --no-default-features --features rust_crypto
• cargo test --features aws_lc_rs
• cargo test --no-default-features --features aws_lc_rs
• wasm-pack test --node --features rust_crypto,getrandom/js
• wasm-pack test --node --no-default-features --features rust_crypto,getrandom/js

Non-goal

• this does not claim to fully resolve Marvin / RUSTSEC-2023-0071

[arckoor]

Let's not depend on a rc version
Once rsa 0.10 releases, cargo should be free to pull in that version, despite us specifying a lower version in the crate (since they seem to be following semver, 0.10 is a patch release, and cargo is free to resolve it. If it has relevant breaking changes, we'll resolve them then)

[EffortlessSteven]

Absolutely. Leaving the RC alone makes sense.

This shook out the rust_crypto-side changes ahead of stable 0.10 and surfaced the wasm/getrandom wrinkle.

Happy to revisit once 0.10.0 is out.