Modify config binding helper registration in clientv2

Config binding helpers (components that add macros to the pystachio
config language) were registered in a way that used self-construction.

In practice, binding helpers need initialization that comes from command-line parameters. With the implicit construction method, we can't provide construction parameters to the helpers.

We need to switches to an explicit construction/registration, instead
of auto-construction when the class is registered. (Interestingly, this
is the way that the documentation on the binding helpers code says that
it works!)

Instead of writing:
FooHelper.register()

We should write:
BindingHelper.register(FooHelper())

Which makes it possible to do:
BindingHelper.register(FooHelper(url=bar))