fix: don't error on invalid i18n tags in discovery

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

pkg/client/client_test.go

@@ -31,7 +31,7 @@ func TestDiscover(t *testing.T) {
 			wantFields: &wantFields{
 				UILocalesSupported: true,
 			},
-			wantErr: true,
+			wantErr: false,
 		},
 	}
 	for _, tt := range tests {