Add Placeholder implementation for an AMD-attested Oak client

[rbehjati]

This PR:

• Adds placeholder implementation for AmdAttestedOakClient, and AmdAttestationReport.
• Moves classes related to Evidence into a separate package.
• Removes the abstract builder in OakClient, as a good implementation of it would require too many type parameters, and would not do much really.

Notable next steps:

• Implement Rekor logEntry verification and verify in EndorsementEvidence.
• Unifying the Rust and Java implementation. It is probably best to do this after updating the interactive remote attestation implementation.

Ref #3066