implement sybil-resistant faucet for KSM

[brenzi]

As a first version of the faucet described in encointer/encointer-parachain#172, we can implement the logic for our native token, the KSM

This avoids all complexity that comes with MultiAsset/MultiOrigin XCM

In order to "burn" used reputation, we probably should use this simple generic pallet in #303 where each instance of a faucet is a purpose which only allows a single use of a proof of attendance