Add organization support to Mixpanel

[tienlethanh]

Implements nested metadata structure for user and organization tracking in MixpanelPublisher.

• Add support for organization metadata alongside user metadata
• Add comprehensive test coverage for all metadata scenarios

Metadata format:

$metadata = [
    'user' => [
             'id' => 'USERID', 
             'properties' => [...]
],
    'organization' => [
             'id' => 'COMPID', 
             'properties' => [...]
]];

[princejohnsantillan]

@tienlethanh implementation looks correct to me. But let's create the publisher class in bw-api land instead. Put it in app\Narratives\Publishers and update the config file to use that MixpanelPublisher instead of the one in this package instead.

'mixpanel' => [
    'class' => \App\Narratives\Publishers\MixpanelPublisher::class,
    'options' => [
        'token' => env('MIXPANEL_PROJECT_TOKEN'),
],

[princejohnsantillan]

This is because implementation is specific to bw-api business logic.