feat: add health check action

[dvikan]

allows for easier programmatic health inspection of an rss-bridge instance.

could later expand with more health data and also check whether the cache is working properly etc.

example:

$ curl -i http://rssbridge.localhost/?action=health
HTTP/1.1 200 OK
Server: nginx/1.18.0
Date: Tue, 04 Jul 2023 10:06:30 GMT
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive

{
    "code": 200,
    "message": "all is good"
}