Add shared state for environment document between edge proxy instances

Currently each Edge Proxy instance maintains a local cache of the environment document, and polls the core API itself.

As part of this issue, we'd like to centralise the environment document, ideally without sacrificing performance.

This issue is closely related to a couple of other issues:

- https://github.com/Flagsmith/edge-proxy/issues/195 - without shared state, implementing this issue means that each Edge Proxy instance needs a connection to the Core API
- https://github.com/Flagsmith/edge-proxy-rs/issues/14 - whatever persistence layer we arrive at for this issue, will likely be re-usable for the other (or vice versa).

Start by reading the Edge Proxy issue alongside Flagsmith/edge-proxy issue 195 and Flagsmith/edge-proxy-rs issue 14. Compare the current per-instance environment-document cache and Core API polling described here, then investigate persistence options that centralize the document without sacrificing performance. Done means the shared-state approach is defined and addresses reuse for the related issue.

Written by the indexing model from the issue text.