Support Batch fee handling

We need the batch builder to process the transaction fees. This involves:
- define the accepted batch fee assets (this issue assumes just one asset, the native asset, as a starting point)
- filtering out tx's which don't carry a fee note at all (`TX_FEE`)
- ensure the fee asset carried is accepted by the builder
- ensure the fee amount covers the tx exec cost
- consume the fee note for itself (batch builder's own account)

For an immediate follow-up, we should extend the list of accepted assets to other assets, and define conversion mechanics.

For future follow-ups, once block kernel implements fee handling, we also need the batch builder to pay enough fees to the block builder to cover the costs of the batch. For now though, the batch builder will just consume all the fees into its own account and not pass them on further.