Add cardano-testnet changelog entry

Author: carbolymer

cardano-testnet/changelog.d/20260220_120000_mgalazyn_add_grpc_interface.md

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+### Added
+
+- Added `--enable-grpc` flag to `cardano-testnet` to enable the gRPC interface (via `cardano-rpc`) when starting a testnet.
+- Added `cardanoEnableRpc` field to `CardanoTestnetOptions` (default `False`).
+- Added `nodeRpcSocketPath` helper to `Testnet.Types` for deriving the gRPC socket path from a node's socket path.
+
+### Tests
+
+- Added integration tests for the gRPC interface: `hprop_rpc_query_pparams` verifies protocol parameters and UTxO queries over gRPC, and `hprop_rpc_transaction` verifies transaction submission over gRPC and confirms the transaction lands on-chain.