Add OutputSweeper utility persisting and sweeping spendable outputs

[tnull]

We add a OutputSweeper utility that allows to track the state of spendable output descriptors as emitted by Event::SpendableOutputs.

To this end, the OutputSweeper persists the necessary information in our KVStore and regularly tries to sweep the spendable outputs, removing them after reaching threshold confirmations, i.e., ANTI_REORG_DELAY.

This is a generalized version of the module previously added to LDK Node: lightningdevkit/ldk-node#152.

TODOs:

• Add test coverage (likely in BackgroundProcessor tests)