Add support for an optional index manager service.

[grugnog]

This is relatively experimental and adds support for https://www.drupal.org/project/indexer. Once we have more experience with this in production, some clearer documentation on the DKAN side and a stable release of indexer, we will add it as a dependency to composer.json.

QA Steps

• Apply patch
• Test DKAN imports datastores normally without indexer module
• Install indexer module
• Test DKAN imports datastores normally with default indexer module (no-op) configuration

[dafeder]

Need clearer instructions.. handing back to @grugnog for now.

[grugnog]

@dafeder added some simplified QA instructions

[dafeder]

QA steps confirmed. Import with indexer enabled does throw a warning:

[warning] Invalid argument supplied for foreach() IndexManager.php:73

but this is coming from the indexer module which is still a WIP. Comfortable merging with an "experimental" disclaimer.