Default to non-zero peg-in fee

[dpc]

To prevent abuse, and (in future) move the cost of spending pegged-in UTXOs from the spender to the creator of UTXO, mints should always charge a peg-in fee.

https://github.com/fedimint/fedimint/blob/5070824f364ae40d35d148422dc3f73ffe68d6a2/modules/fedimint-wallet-common/src/config.rs#L109

Probably something like 1000sats per-peg-in should be OK.

We need to expose ability to customize it and discover it by the client applications, so it can be integrated in the UX.