HTTP: added basic URI rewrite.

This commit introduced the basic URI rewrite. It allows users to change request URI. Note the "rewrite" option ignores the contained query if any and the query from the request is preserverd.
An example:
"routes": [
    {
        "match": {
            "uri": "/v1/test"
        },
        "action": {
            "return": 200
        }
    },
    {
        "action": {
            "rewrite": "/v1$uri",
            "pass": "routes"
        }
    }
]

Reviewed-by: Alejandro Colomar <alx@nginx.com>

docs/changes.xml

@@ -32,6 +32,12 @@ NGINX Unit updated to 1.30.0.
          date="" time=""
          packager="Nginx Packaging <nginx-packaging@f5.com>">
 
+<change type="feature">
+<para>
+basic URI rewrite support.
+</para>
+</change>
+
 <change type="feature">
 <para>
 added conditional logging of route selection for HTTP requests.