Add ability to request metadata for multiple assets in one call

[jhedstrom]

The DAM API supports passing in multiple asset IDs to the metadata endpoint. (example https://apiv2.webdamdb.com/assets/2342322,5342344/metadatas/xmp)

The current code assumes only a single asset ID though:

    $response = json_decode((string) $response->getBody());

    $metadata = [];
    foreach ($response->active_fields as $field) {

resulting in errors if multiple IDs are passed (such as '123,45,67') as the response format is different if more than a single asset ID is passed.

The change here could either be a completely new method that takes an array of IDs and internally concatenates them with a comma, or refactor the current method to work with both single IDs or comma-concatenated list of IDs.