Add missing tags_api mapping for get_trending_tags method

As described in devportal, tags_api.get_trending_tags should be an alias to condenser_api.get_trending_tags.

I suspect while offloading tags API calls to Hivemind, this mapping is forgotten.

I've tested the patch on my API node, and it works as expected:

$ curl - s--data '{"jsonrpc":"2.0", "method":"tags_api.get_trending_tags", "params":{"start_tag":"hive","limit":1}, "id":1}'
https: //hived.emre.sh
    {
        "jsonrpc": "2.0",
        "result": [{
            "name": "hive-189157",
            "comments": 139,
            "top_posts": 38,
            "total_payouts": "190.898 HBD"
        }],
        "id": 1
    }