Get block header, with option to get array of TxIDs in the block

[Yoav Tock]

In order to implement a "delivery/notification service" in the SDK, we need a way to monitor the stream of TXs that where committed.
When getting a block header, using

GetBlockHeader     = "/ledger/block/{blockId:[0-9]+}"

add the option to get the augmented block header that includes TxIDs in an array that is keyed by the index, in addition to the *type.BlockHeader.