Add country and request ID headers to backend requests

FlowGuard proxies now add two headers to requests sent to your backend:

• FG-IPCountry carries the client's two-letter country code, or XX when it's unknown.
• FG-Stream carries the request ID that matches the stream ID in request logs.

Use them to localize responses or to find the log entry for a request. Both are on by default and can be turned off under Upstream headers when editing a server. Copies of these headers sent by clients are always removed. Requires proxy v0.26.0 or newer.