Bulk endpoint for /utxo/withPool/byId

[pulsarz]

Currently when fetching the entire mempool the inputs of those transactions only contain the boxId.

When trying to parse the entire mempool (in my case for SigmaExplorer) we need to request all input boxes one by one currently.
Considering transactions can contain many hundreds of input boxes this makes it impossible to retrieve them all in a timely way.

A new endpoint for fetching unspent boxes by POST'ing an array of boxIds would speed up things enormously.
Considering this only does a UTXO and mempool scan this should be fairly light load on the node side.

We could also consider this for other such endpoints if there is any demand.