Add belongs_many_many support to search field scaffolding

[LeviWB]

Dataobjects with $many_many fields can be easily added to a search index by adding the field in the init() method of a search index

e.g.

public function init()
{
    $this->addClass(MyClass::class);

    $this->addFilterField('MyManyManyField.ID');
}

It would be handy if this also worked with $belongs_many_many fields.

I've had a look into this and one way to support this would be to make the following change to the fieldData() method in SilverStripe\FullTextSearch\Search\Indexes\SearchIndex:

Replace

$relationList = Config::inst()->get($dataclass, 'many_many', Config::UNINHERITED);

with

$relationList = array_merge(
    Config::inst()->get($dataclass, 'many_many', Config::UNINHERITED) ?? [],
    Config::inst()->get($dataclass, 'belongs_many_many', Config::UNINHERITED) ?? [],
);

I'm happy to create a pull request for this if it sounds reasonable.

[GuySartorelli]

New features aren't being added to this module by the CMS Squad, as it isn't commercially supported for the Silverstripe CMS 5.x release line.

If you'd like to implement this as a PR it's possible someone will see it and review it, but there are no guarantees.

If you'd like to get access to maintain this module you can email community@silverstripe.com and request access.

Alternatively you could look at using https://codeberg.org/Firesphere/silverstripe-solr which is maintained by a community member and has a migration path for projects that are currently using this module.

[michalkleiner]

Happy to review a PR ;-)