Support non-direct peer configuration

[wilsonianb]

In order to support the connector routing packets to a destination that is a peer of a peer (no parent / child relationships), it should be possible to add the non-direct peer in such a way that packets are sent to the direct peer and the direct peer's liquidity account is used.

With the current peer model, the non-direct peer can be added with its staticIlpAddress and the direct peer's outgoingToken/outgoingEndpoint, but a liquidity acount for the non-direct would be used (which is not desirable) https://github.com/interledger/rafiki/blob/main/packages/backend/migrations/20211026133430_create_peers_table.js

[mkurapov]

• Related to https://github.com/interledger/rafiki/issues/2229