feat: expose txHash and msgIdx in ctx

[mhofman]

Description

In order to support Agoric/agoric-sdk#7102 and future shared context between golang and JS, we need to build a unique context for actions pushed to cosmic-swingset on the cosmos side.

This change adds the "transaction hash" and "message index" within that transaction to the message context, which allow to uniquely identify actions and link them to their origin while pushing them to JS.

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[michaelfig]

LGTM! @JimLarson may want to opine before merging.

[JimLarson]

LGTM