GET /v5/channel/:id/get-leaf route + tests

[simzzz]

No description provided.

[simzzz]

TODO:

• Fix te failing test + maybe find a better testing approach
• Documentation for route
• Code examples for GetLeafResponse

[elpiel]

Also take into account what we've discussed on slack how to set the earner/spender sides.
enum LeafFor { Spender, Earner} and add it to the respective route as an Extension

[elpiel]

Look pretty good. A few polishing touches on Errors remain and a few tests regarding:

• no ApproveState - 404
• no NewState (but has approvestate) - 400
• no spender/earner - you could use the same address and call the function twice