Add Oblivious HTTP (OHTTP) client for iOS

Using the ohttp-rs and bhttp-rs Rust libraries to perform underlying encoding
and encryption. This is exposed to Swift using URLRequest/Response types that
are typical for the platform.

Note that only iOS is targetted since Gecko projects have a builtin OHTTP
solution within Necko.

This use currently using the Rust Crypto libraries for underlying HPKE support,
but this should be changed to NSS or OpenSSL in future.

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 [Full Changelog](In progress)
 
+## General
+
+### ✨ What's New ✨
+
+- Added an OHTTP client library for iOS based on `ohttp` Rust crate ([#5749](https://github.com/mozilla/application-services/pull/5749)). This allows iOS products to use the same OHTTP libraries as Gecko-based products.
+
 # v117.0 (_2023-07-31_)
 
 ## General