Allow fetching the UTxO including mempool updates

[MuesliSwapTeam]

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This is a first part of a line of contributions based on the Catalyst Proposal Open Transaction Chaining tooling to speed up Cardano dApps - By MuesliSwap.

The PR builds on top of https://github.com/input-output-hk/nami/pull/885 to enable building transaction making usage of knowledge about the current mempool - thus being able to consume UTxOs that are not officially on-chain yet and ignoring such inputs that will be consumed by transactions currently waiting to be confirmed.