Add support of stateful http checks

[umputun]

The idea is to allow checkin state's change via http. In other words we want to keep track of some predefined response element (from response body) like foo=123 and ensure the value for this key is changing (or not changing) according to our expectations.

In a way it is similar to what we already do for file provider, however instead of file size it will be using a value from http response body

[dimkalinux]

Any hint how "predefined response element" may be set in config? And how to extract it from response, regexp not very easy in this case?

Because file size is hardcoded.

[umputun]

could be just dot notation for a field we care about, i.e. "path=foo.bar"