[todo] Swap endpoint

[jordaaash]

Is your feature request related to a problem? Please describe.
Currently the only endpoint is transfer which will pay Octane with an SPL token. This requires the operator of Octane to periodically replenish their wallet with SOL.

Additionally, transferring SPL tokens is nice, but doesn't easily give the user a way to get SOL, which they could use for their own transactions that may require more flexibility.

Describe the solution you'd like
An endpoint that allows swaps from SPL tokens to SOL -- for example, using Serum's upcoming Swap instruction or an AMM -- could allow Octane and the user to receive SOL.