Add an example for Verifying GCP attestations inside nitro enclaves

[chargarlic]

Adds an example to show how to verify GCP Confidential compute attestations (JWT) inside aws nitro enclaves. The response includes a signed response from the enclave which can be used as an extension to verify GCP attestations onchain in Sui

[joyqvq]

this should be in the examples/ folder for rust and reuse the template with feature flag e.g. gcp-verify-example, as much as possible, instead of copy pasting everything over.