Add optional ICU support for user search (#14464)

Fixes #13655

This change uses ICU (International Components for Unicode) to improve boundary detection in user search.

This change also adds a new dependency on libicu-dev and pkg-config for the Debian packages, which are available in all supported distros.

debian/changelog

@@ -1,3 +1,10 @@
+matrix-synapse-py3 (1.74.0~rc1) UNRELEASED; urgency=medium
+
+  * New dependency on libicu-dev to provide improved results for user
+    search.
+
+ -- Synapse Packaging team <packages@matrix.org>  Tue, 06 Dec 2022 15:28:10 +0000
+
 matrix-synapse-py3 (1.73.0) stable; urgency=medium
 
   * New Synapse release 1.73.0.

debian/control

@@ -8,6 +8,8 @@ Build-Depends:
  dh-virtualenv (>= 1.1),
  libsystemd-dev,
  libpq-dev,
+ libicu-dev,
+ pkg-config,
  lsb-release,
  python3-dev,
  python3,