Set proxy_ignore_headers to proxy-cache config block

### Description

We'd like to be able to set the proxy_ignore_headers in the [proxy-cache config block](https://github.com/apache/apisix/blob/master/apisix/cli/ngx_tpl.lua#L773)

Use case: We want to cache responses no matter what's in the Cache-Control header so we need to set
`proxy_ignore_headers Cache-Control;`

Ideally this should be per route, as we configure the proxy-cache plugin per route. Our deployment relies in K8S APISIXRoute resources for configuration so I assume this will affect some other layers apart from the LUA tpl.

We have tested this in a live system by tweaking the nxg_tpl.lua file and reloading the config. After reloading the config the cache started HIT'ing just fine, whole before it was always MISS'ing

Start at apisix/cli/ngx_tpl.lua around line 773 and trace how the proxy-cache plugin and Kubernetes APISIXRoute resources produce configuration. Check the existing reload behavior described in the issue, then determine the configuration path needed for per-route proxy_ignore_headers. Done means Cache-Control responses can be cached for the configured route and produce cache HITs after reload.

Written by the indexing model from the issue text.