walletkit: add custom-anchor capability discovery

[darioAnongba]

Summary

• Add SDK-owned custom-anchor capability DTOs with supported, unsupported, and unknown states.
• Add structured unsupported-capability errors for pre-signing capability checks.
• Expose equivalent static tapd capability profiles through gRPC and REST WalletKit clients and the high-level wallet.
• Document the three-state capability model in the custom-anchor design doc.

Refs #152

Validation

• gofmt -w custom_anchor_capabilities.go custom_anchor_capabilities_test.go
• go test . ./grpc ./rest
• 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