fix(model-loading): keep track of open GRPC Clients (#3377)

Due to a previous refactor we moved the client constructor tight to the
model address, however that was just a string which we would use to
build the client each time.

With this change we make the loader to return a *Model which carries a
constructor for the client and stores the client on the first
connection.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>