Refreshable HTTP getters

[ppcad]

Right now HTTP getters don't get refreshed if the result at the HTTP endpoint changes.
To achieve a refresh behavior HTTP getters could periodically poll if their values have changed.
Once a change has been detected, HTTP getters could communicate this so that the new values can be used.

The refresh period could be tied to the configuration refresh interval and it might trigger a configuration reload if a HTTP getter was used in a configuration file.
Futhermore, HTTP getters could communicate changes directly by calling a callback function that has been provided to them on creation (in code).
Alternatively or additionally, a detected change could trigger a reload, even if it's not part of a configuration file, instead of using a callback. However this is expensive if a value changes often and if no full reload would be necessary.

This functionality is not always required so it should be optional.