follow convention to use "_supported" suffix on AS metadata

Issue #88 resolved
Brian Campbell created an issue

In a comment on issue #69 @jeps raises this good point:

I think the convention set by RFC8414 is use "_supported" suffixes on certain parameter names. If we follow this convention then the name of backchannel_token_delivery_modes parameter should probably be backchannel_token_delivery_modes_supported.