feat: write X509 KeyInfo during signing

[polaz]

Problem

Signing templates can emit an empty <KeyInfo/> placeholder, but SignContext
does not yet have a writer hook to populate it. SAML IdP signing needs embedded
X.509 certificate material so verifiers can resolve the public key from the
signature document.

Solution

Implement P2-018 from the roadmap:

• Add a KeyInfoWriter trait used by signing.
• Add an X.509 certificate writer that emits <X509Data>/<X509Certificate> with
  base64 DER certificate content.
• Wire SignContext so sign_template() and sign_with_builder() populate the
  direct <Signature>/<KeyInfo> placeholder when a writer is configured.
• Preserve XMLDSig child order: SignedInfo, SignatureValue, optional
  KeyInfo, then Object*.
• Keep KeyInfo outside SignedInfo so canonicalized signed bytes and digest
  computation are unaffected by writer output.

Acceptance Criteria

• Signing with a builder that includes key_info(true) can write an embedded
  X.509 certificate into <KeyInfo>.
• Signed output verifies through the existing DefaultKeyResolver using the
  embedded certificate.
• Existing signing paths without a writer remain unchanged.
• Missing or duplicate direct signing <KeyInfo> placeholders fail explicitly
  when a writer is configured.
• cargo fmt -- --check, cargo clippy --all-targets --all-features -- -D warnings,
  cargo nextest run --all-features, and cargo test --doc --all-features pass.

Estimate

2d: 0.5d trait/API, 0.5d XML mutation, 0.5d X.509 writer, 0.5d tests and docs.