Release v1.2.0

IMPORTANT: This is a breaking change.

• Health check callback now must return a string instead of an object and the value will be send directly as the output.

Reason for this change

Previously, the user had to return an object or struct, and ensure it contained a copy of the internal application state, usually protected by a mutex to avoid concurrent reading and writing.

If slices, maps or pointers were present, a deep copy was required in order to avoid accessing shared data out of the mutex.

Now, to avoid confusion, the callback must, for e.g., encode a struct as a JSON string and return it.