Expose a way for indexing strategies to get the commit evidence of the TxID.

[plietar]

Is your feature request related to a problem? Please describe. See #4247

Describe the solution you'd like In order to compute a receipt, an indexing strategy needs the signature, the inclusion proof and the commit evidence. The first two are covered by #4247.

Because indexing strategies are only called with globally committed transactions, I believe it would be safe to pass that evidence as a new argument to handle_committed_transaction, or alternatively provide a way to compute it lazily if doing it for every transaction is too expensive.