optionally keep indexes of endpoints in metadata

Created by: melanger

Allow keeping endpoints at the same indexes as in metadata, for SPs which rely on the indexes instead of full addresses. Example config (after merge):

// ...
'flatfile2internal' => [
        // ...
        'usesIndexes' => 'entityid',
],
'importTransformers' => [
        [
            'class' => '\\SimpleSAML\\Module\\perun\\transformers\\AttributeAlter',
            'attributes' => ['usesIndexes'],
            'config' => [
                'pattern' => '/^https:\/\/sp\.tshhosting\.com\/shibboleth$/',
                'replacement' => '',
            ],
        ],
        [
            'class' => '\\SimpleSAML\\Module\\perun\\transformers\\LogicalNot',
            'attributes' => ['usesIndexes'],
            'config' => [],
        ],
        [
            'class' => '\\SimpleSAML\\Module\\perun\\transformers\\EndpointIndexMap',
            'attributes' => ['assertionConsumerService', 'usesIndexes'],
            'config' => ['defaultBinding' => 'HTTP-POST', 'shortNames' => true],
        ],
        // ...
],