Add DSR support

[thowar2]

Using CHAI, supporting gasless DSR is as easy as supporting mcDAI.

https://chai.money/

[datgrog]

https://github.com/dapphub/ds-dach is a solid version of gasless point of view with CHAI support (discover this yesterday).
I used gasless and add chai support myself and ending up quite the same as ds-dash so, looks like cross validate in a way.