Add hook to include custom variables by default when searching under Overview entries

[oskarnrk]

refs #1045

The hook can be implemented in multiple ways. For example:

Add to run.php

$this->provideHook('Icingadb/CustomVarsRetriever', 'ExampleCustomVarsRetriever');

ExampleCustomVarsRetriever.php

<?php

namespace Icinga\Module\Icingadb\ProvidedHook;

use Icinga\Module\Icingadb\Hook\CustomVarsRetrieverHook;
use ipl\Orm\Model;

class ExampleCustomVarsRetriever extends CustomVarsRetrieverHook
{
    public function retrieveCustomVars(Model $model): array
    {
        switch ($model->getTableName()) {
            case 'host':
                return ['host.vars.hostfoo', 'host.vars.hostbar'];
            case 'service':
                return ['service.vars.servicefoo', 'service.vars.servicebar'];
            default:
                return [];
        }
    }
}

Some implementation suggestions could be:

• retrieve the custom vars and filter letting pass only the ones that actually exist
• provide multiple hooks to implement different logic for a specific model (it's given as input)
• save the wanted custom variables in a config.ini file and retrieve them from it

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[oskarnrk]

Thank you for your pull request. Before we can look at it, you'll need to sign a Contributor License Agreement (CLA).

Please follow instructions at https://icinga.com/company/contributor-agreement to sign the CLA.

After that, please reply here with a comment and we'll verify.

Contributors that have not signed yet: @oskarnrk

I signed the ICLA, thanks in advance.