Add a default impl for 'DebugSecret' trait

[nbigaouette]

By using a default implementation of the trait's function
'debug_secret()', downstream users don't have to define
the function themselves. They can simply:

    impl DebugSecret for MyType {}

and the static string returned will be the default one.

Downstream users can still add a full 'debug_secret()'
definition to the trait impl to change the string used.

[tony-iqlusion]

Thank you!