Use TAV for SNP attestation verification

[cjen1-msft]

Stack

• Depends on Vendor TAV 1.0.3 #8067. Do not merge this PR before the vendor PR.
• Integration-only comparison: cjen1-msft/CCF@tee-attestation-verification-tav...tee-attestation-integration-tav

Summary

• add the OpenSSL-backed TAV FFI crate to the ccf-rs umbrella
• replace PAL SNP certificate-chain, report-signature, and TCB verification with tav_verify_snp_attestation
• retain the existing CCF SNP report type, report accessors, and CCF-specific policy checks
• defer migration of report ownership and accessors to a later change

Testing

• Rust 1.90 locked release umbrella build and tav_verify_snp_attestation symbol check
• snp_attestation_test (12 cases, 49 assertions)
• ccf target build
• CMake and C++ formatting
• copyright, ASCII, public include, and diff checks
• independent verifier correctness and MVP scope reviews