Add the url for getting the lastest version number to health

This is related to https://github.com/skycoin/skycoin/issues/2043

As the wallet is going to get the number of the lastest version from a custom URL, to know if an update is available, that URL should be added to the `health` API endpoint. This is needed mainly for being able to easily configure the Fiber coins.

To clarify:

The version is downloaded from https://version.skycoin.net/skycoin/version.txt. This issue is requesting that that URL be configurable.

Start by tracing the health API endpoint and the existing version-check configuration described in related issue #2043. Confirm how the endpoint currently reports health and where Fiber coin settings are configured. Done means the version URL is configurable and exposed through health, with behavior verified for the default URL https://version.skycoin.net/skycoin/version.txt.

Written by the indexing model from the issue text.