Req cacher plugin

Use cache if it exists and not expired, otherwise update cache with response in status 200.

Mainly used in some rarely changed or slowly API requests.

This is not the default :cache request-step but some code from it

• https://github.com/wojtekmach/req/blob/main/lib/req/steps.ex#L639

Support proxy value

• :cacher boolean()
• :cacher_dir String.t() (optional)
• :cacher_fresh_days integer() (optional), if set <= 0 means always expired