feat: complete SigningKey public key abstraction

[polaz]

Problem

The signing-key abstraction currently exposes sign() and raw SPKI bytes, but public-key metadata is still ad hoc. KeyInfo writers and future signing APIs need a structured way to inspect the public key corresponding to the private signing key.

Current code analysis

• src/xmldsig/sign.rs defines SigningKey::sign() and SigningKey::public_key_spki_der().
• RSA, ECDSA P-256, and ECDSA P-384 signing keys implement the trait.
• X509CertificateKeyInfoWriter uses raw SPKI DER only to reject certificate/key mismatches.
• There is no structured public-key info return value for callers or future KeyInfo writers.

Solution

• Add structured public-key metadata to the signing-key abstraction.
• Preserve object safety for SignContext and KeyInfoWriter.
• Return enough metadata to distinguish RSA and ECDSA keys and expose SPKI DER without reparsing private keys.
• Keep existing signing behavior unchanged for RSA-SHA256/SHA384/SHA512 and ECDSA P-256/P-384.
• Add tests for public-key info on RSA, P-256, and P-384, unsupported algorithm errors, and certificate/key mismatch behavior after the API change.

Acceptance Criteria

• SigningKey exposes signing plus structured public-key information.
• RSA/P-256/P-384 concrete signing keys implement the full trait.
• X509CertificateKeyInfoWriter uses the structured public-key info path, not an ad-hoc raw method.
• Existing sign/verify round-trips continue to pass.
• cargo fmt -- --check, cargo check --all-features, cargo clippy --all-targets --all-features -- -D warnings, cargo nextest run --all-features, and cargo test --doc --all-features pass.

[coderabbitai]

🔗 Related PRs

#53 - feat(xmldsig): parse X509Certificate with x509-parser [merged]
#66 - feat(xmldsig): add key resolver configuration [merged]
#68 - feat(xmldsig): resolve RSAKeyValue keys [merged]
#70 - feat(xmldsig): resolve ECKeyValue keys [merged]
#81 - feat(xmldsig): write x509 signing key info [merged]

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