Add changelog entry for --nodes flag

Author: palas

cardano-testnet/changelog.d/20260506_035740_palas_testnet_specify_node_bin_per_node.md

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+### Added
+
+- Added `--nodes` flag to specify node roles (SPO/relay) and custom `cardano-node` binaries per node.
+  Example: `--nodes spo,spo:node-bin=/path/to/bin,relay,relay`.
+
+### Changed
+
+- Renamed `NodeOptions` to `NodeWithOptions` and `TestnetNodeOptions` to `TestnetNodesWithOptions`
+  (exported from `Testnet.Start.Types` and `Cardano.Testnet`). The new types include a `nodeBin` field for
+  specifying a per-node `cardano-node` binary.