Add Cache Support for Deepseek Provider

[danividalg]

Hi,

As we can see here deepseek API allows use of cache
https://api-docs.deepseek.com/quick_start/pricing/

But in the configuration it say "Does not support prompt caching"

Please add cache support for deepseek provider.

Thanks a lot :)

[hannesrudolph]

Done in d2c2029