feat(lexicon): configurable default value

[ArtificialOwl]

This feature require that the config key is listed in the Config Lexicon.

• add a new system config key 'lexicon.default.userconfig' that overwrite default userconfig value:

   'lexicon.default.userconfig' => ['core' => ['lang' => 'fr']],

• add a new system config key 'lexicon.default.userconfig.enforced' that lock config value to default:

   'lexicon.default.userconfig.enforced' => ['core' => ['lang' => true]],

(enforced default value will be chosen from the one stored in config.php, the one set in the lexicon then to the one from the code)

• add a CoreConfigLexicon for 'core' config keys

also some improvement in the resource management

• extract default value only if needed,
• lazy conversion of default to string.