[amp-geo] Support iOS 15 Region level IP

### Description

`amp-geo` will no longer be able to use the users' IP address on Safari to determine their country/state code. This will be large problem since `amp-geo` needs to be able to supply country code information for processes such as consent.

Safari proposes [`Sec-CH-GeoHash`](https://tfpauly.github.io/privacy-proxy/draft-geohash-hint.html) as a solution where IP addresses will be grouped by region, and the GeoHash information (stored as lat and long) will be accessible as a [client hint](https://wicg.github.io/client-hints-infrastructure/).

Client hint headers are only sent per-domain, and on second request.

The [following document](https://docs.google.com/document/d/1oJWJ7slgxs_hdrGvgjbUAVTlyPXe0aQf3rKdbMkCFhQ/edit?usp=sharing) outlines the problem, potential solutions, and the first step (experiment with serving both the HTML and JS from the publishers CURLS domain).

### Alternatives Considered

Outlined in doc

### Additional Context

_No response_

Start with the amp-geo entry point and read the linked design document, focusing on the proposed iOS 15 region-level IP and client-hint experiment. The issue does not name files or tests; done would require implementing and validating an agreed solution for obtaining country/state information on Safari.

Written by the indexing model from the issue text.