ExplorerHeader is a trait that represents certain extensions to the
[BlockHeader] that are specific to the Block Explorer API. This trait
allows for the explorer module to be defined and provide an API for any
consuming Block Explorer.
ExplorerTransaction is a trait that allows the Explorer API to be able to
retrieve a namespace id from a transaction. This is necessary for the
Explorer API to be able to display the namespace id for a
TransactionSummary.