feat: Set If-Modified-Since header for environment document requests

[rolodato]

Adds support for Flagsmith/flagsmith#5283

Example log output:

❯ rye run edge-proxy-serve
2025-04-29 19:26:49 [info     ] Loaded configuration from config.json
2025-04-29 19:26:49 [info     ] Loaded configuration from config.json
2025-04-29T22:26:49.666581Z [info     ] Started server process [50212] [uvicorn.error]
2025-04-29T22:26:49.666789Z [info     ] Waiting for application startup. [uvicorn.error]
2025-04-29T22:26:49.701255Z [info     ] HTTP Request: GET http://localhost:8000/api/v1/environment-document/ "HTTP/1.1 200 OK" [httpx]
2025-04-29T22:26:49.701813Z [info     ] Application startup complete.  [uvicorn.error]
2025-04-29T22:26:49.702157Z [info     ] Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit) [uvicorn.error]
2025-04-29T22:26:49.702853Z [info     ] HTTP Request: GET http://localhost:8000/api/v1/environment-document/ "HTTP/1.1 304 Not Modified" [httpx]
2025-04-29T22:26:52.707151Z [info     ] HTTP Request: GET http://localhost:8000/api/v1/environment-document/ "HTTP/1.1 304 Not Modified" [httpx]
2025-04-29T22:26:55.710535Z [info     ] HTTP Request: GET http://localhost:8000/api/v1/environment-document/ "HTTP/1.1 304 Not Modified" [httpx]