Configure `FLAGSMITH_PROXY_URL` per-route

We have seen a customer need to make SAML response URLs usable in environments that block all public access to the Flagsmith API. Other routes, like webhook URLs, may require the same treatment. We can use `FLAGSMITH_PROXY_URL` but it makes the entirety of the API served by the FE container, which requires additional ingress configuration. We should allow FE proxy mode on a per-route basis, with documented defaults for various use cases.