Add API Endpoint for Dust Threshold
We need to add an endpoint to the wallet API that queries the dust threshold of the wallet. This is good for the UI to know so that it can mark which transactions have outputs that are dust outputs, effectively warning the user that the transaction is not going to be counted for its full balance in the total balance.
How the dust threshold is managed is really a wallet implementation decision, which is why we shouldn't just tell people to take the /fee endpoint and multiply by 3.
Like all API endpoint changes, any pull request to add this needs to update doc/API.md, doc/api/wallet.md, and needs to have a test definitively proving that the dust threshold endpoint is displaying sane values and is functioning correctly.