Geo - New geo/site_proxy endpoint to supersede geo/node_proxy

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What / Why

We are currently working to rename the RESTful API for geo_nodes => #369140
Similar to the above rename we should aim to rename the geo/node_proxy as well.

Proposal

1. Determine deprecation strategy of geo/node_proxy
2. Create new geo/site_proxy
3. Update FE references
4. Sunset geo/node_proxy

cc / @sranasinghe @juan-silva