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>

src/nxt_http_parse.h

@@ -127,6 +127,7 @@ nxt_uint_t nxt_http_fields_hash_collisions(nxt_lvlhsh_t *hash,
 nxt_int_t nxt_http_fields_process(nxt_list_t *fields, nxt_lvlhsh_t *hash,
     void *ctx);
 
+nxt_int_t nxt_http_parse_complex_target(nxt_http_request_parse_t *rp);
 nxt_buf_t *nxt_http_chunk_parse(nxt_task_t *task, nxt_http_chunk_parse_t *hcp,
     nxt_buf_t *in);