Auth for OPDS Spec, Basic and Password grant, allowed properties should be clearer

[llemeurfr]

In both Basic and Password grant types, the Authentication Provider may provide custom labels for the login and password.
This should be clear in the spec, with the chosen property names, in sections 3.3 and 3.4.6.
The example in 2.2.3 indicates the property names are login and password in a labels object.

[HadrienGardeur]

What's the best way to handle this in your opinion ? We could provide a table in a non-normative section for instance.