ExplorerHeader should not have the method namespace_ids_for_offset

[Ayiga]

The method namespace_ids_for_offset is intended to get a list of NamespaceIds for a given Transaction offset on a Block Header. Considering implementation details the Block Header doesn't actually have access to this information, based on the discussion here: EspressoSystems/espresso-sequencer#1428 (comment).

Since this method is only utilized for TransactionSummary, TransactionSummary has access to the actual Transaction itself when being constructed, and the implementation details for a Transaction actually has access to its NamespaceId, then we can make this work by creating a trait for the Transaction itself to retrieve its own NamespaceId.

So, we need to remove namespace_ids_for_offset, and create a new trait for a Transaction for use with the Explorer API, that allows us to retrieve the NamespaceId from the transaction itself.