Geo: Read-only Geo secondary sites
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### Problem to solve
Since enabling proxying by default with both unified URL and separate secondary URLs there may be use cases where customers want to maintain the secondary site as a read-only site.
Currently proxying is controlled by a feature flag which is turned on by default.
This feature flag does not entirely stop proxying in all scenarios. (We would also need to implement the ability to disable proxying of Git over SSH requests.)
### Proposal
Proxying should be a configurable setting per Geo site.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
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