Fee sweeper: cow burner

[jubeira]

Implement / fork cow burner as the default burner alternative.
Default burner shall be configurable.

See

• https://explorer.cow.fi/address/0xC0fC3dDfec95ca45A0D2393F518D3EA1ccF44f8b
• docs: https://docs.curve.fi/fees/CowSwapBurner/

[jubeira]

https://github.com/curvefi/curve-burners