Add integration test suite for sigstore (full sigstore stack)

[morri-son]

Description

Add a self-contained integration test suite under bindings/go/sigstore/integration/ using a local kind cluster with a full Sigstore stack.

Infrastructure (via Helm):

• Rekor v1 (sigstore/scaffold): Fulcio, CTLog, Trillian, TUF, TSA
• Rekor v2 (rekor-tiles, POSIX backend)
• Port-forwards for all services to localhost

Test scenarios required:

• Key-based sign/verify (ECDSA, Ed25519)
• Keyless sign/verify (OIDC token via kubectl create token)
• Identity verification (issuer + SAN enforcement)
• TSA integration (required for Rekor v2 — no SETs)
• signing_config.json endpoint discovery
• Custom TUF mirror verification
• Minimal bundle (no Rekor/Fulcio)
• Tampered digest detection

Done Criteria

• task kind/setup deploys Rekor v1 + v2 + Fulcio + TSA + TUF in kind
• All test scenarios pass against both Rekor v1 and v2 backends
• task test/integration runs the full suite
• Setup/teardown scripts are idempotent
• Code reviewed by other team members
• CI pipeline integration documented or CI job added
• Successful demonstration in Review