Previously you couldn't have a brokerd backend which defined
`.trades_dialogue()` but which could also indicate that the paper
clearing engine should be used. This adds that support by allowing the
endpoint task to return a simple `'paper'` string, in which case the ems
will boot a paperboi.
The obvious useful case for this is if you have a broker you want to use
but do not have actual broker credentials setup (yet) with that provider
in your `brokers.toml`; demonstrated here with the adjustment to
`kraken`'s startup to no longer raise a runtime error B)
769b292dca