Add "set_fee_schedule" to channel API

[lullis]

Relates to #5011.

api/python.py already provides some methods to set attributes from a channel. It also needs to have a method to create a FeeSchedule.

It needs to be able to create FeeSchedule all of its possible attributes:

• Flat fee
• Proportional fee
• Imbalance fees

[karlb]

Part of/duplcate of #5011