API needs an endpoint to submit a package of txs

Currently there is the `POST /tx` endpoint where you can submit a tx. With Bitcoin Core 28 releasing package relay in a limited form, it is possible using Core to submit and relay packages of transactions. More specifically, it is possible to relay a pair of txs of which either of them would not be accepted into the mempool alone.

This means that submitting the txs one by one into the `/tx` endpoint does not work and the API needs to be able to accept packages of multiple (currently just 2) transactions.

Maybe `POST /tx` can support a body of multiple lines, or of a JSON array, or a new endpoint `/txs` can be added.