feat: add SPIFFE/SPIRE bridge

[jamilahmadzai]

Closes #12

Summary

• Adds a Go SPIFFE/SPIRE bridge example for APort identity federation.
• Parses SPIFFE IDs into trust-domain and workload selector context.
• Verifies the mapped APort agent id against a configurable APort policy.
• Includes a CLI example, HTTP verifier, federation flow, documentation, and tests.
• Marks the SPIFFE/SPIRE bridge active in the root README.

Testing

docker run --rm -v "$PWD":/work -w /work golang:1.22 sh -lc '/usr/local/go/bin/gofmt -w cmd/bridge/main.go internal/bridge/*.go && /usr/local/go/bin/go test ./... && /usr/local/go/bin/go vet ./...'
git diff --check

Payout

Payment method: PayPal jamilurrehman722@gmail.com.