REST API: SetFrequency

[jbride]

HTTP Put API to set frequency on a board

Path = "/api/v1/board/{serial}/frequency",
Response: JSON representation of SetFrequencyResponse:

[rkuester]

MIP-0001 makes this a write to the board's target_frequency_mhz setpoint in the unified tree, rather than a dedicated endpoint. Folding into #57; closing as superseded. Thanks for the spec.