This changes the use of `[]language.Tag` to
`oidc.Locales` in `DiscoveryConfig`.
This should be compatible with callers that use
the `[]language.Tag` .
Locales now implements the `json.Unmarshaler` interface.
With support for json arrays or space seperated strings.
The latter because `UnmarshalText` might have been implicetely called
by the json library before we added UnmarshalJSON.
Fixes: #406
