custom-anchor: add transfer package DTO

[darioAnongba]

Summary

• Add CustomAnchorTransferPackage as the persistable post-commit DTO for advanced custom-anchor flows.
• Include SDK-owned summaries for committed inputs, outputs, locked UTXOs, proof updates, and publish/log retry metadata.
• Add validation and deep-copy helpers so callers can snapshot committed state before external signing or broadcast.
• Tie the design doc's transfer-package boundary to the new root SDK DTO.

Refs #151

Validation

• gofmt -w custom_anchor_package.go custom_anchor_package_test.go
• python3 line-length check for custom_anchor_package.go and custom_anchor_package_test.go
• go test .
• go test ./rest ./grpc
• go test ./...
• go test -tags=itest -run '^$' ./itest/...
• git diff --check
• GOWORK=off go run -modfile=tools/go.mod github.com/golangci/golangci-lint/v2/cmd/golangci-lint run -v