support multiple cache-valid(ity)/proxy_cache_valid directives

[Paul Collins]

Currently cache-valid(ity) has to be a single item:

 - cache-valid: 200 1h

or

 - cache-valid: 200 401 1h

which maps to

proxy_cache_valid 200 1h;

or

proxy_cache_valid 200 401 1h;

Ideally we could also supply a list so that, say

 - cache-valid:
   - 200 301 1h
   - 401 15m

would generate

proxy_cache_valid 200 301 1h;
proxy_cache_valid 401 15m;

which seems to be what is needed to get different lifetimes: https://docs.nginx.com/nginx/admin-guide/content-cache/content-caching/#limiting-or-disabling-caching