backend(rwkv): python rwkv backend

[mudler]

Is your feature request related to a problem? Please describe.

We have rwkv support with rwkv.cpp with the go-rwkv binding, however that might not be in par with rwkv changes

Describe the solution you'd like
Support also https://pypi.org/project/rwkv/ alongside rwkv.cpp

Describe alternatives you've considered
keep rwkv support as-is

Additional context

Eagle 7b and Raven 13b now got very good benchmarks !

https://twitter.com/RWKV_AI/status/1751797147492888651

Part of #1126