🔗 Add ServiceIntegration Model for User Credentials

[pr-pilot-ai[bot]]

The sub-task to define integration credentials has been completed. A new Django model ServiceIntegration has been added and connected to the PilotUser model in accounts/models.py.

Here is the new ServiceIntegration model:

class ServiceIntegration(models.Model):
    service_name = models.CharField(max_length=100)
    api_key = models.TextField()
    api_secret = models.TextField()
    user = models.ForeignKey('PilotUser', on_delete=models.CASCADE, related_name='service_integrations')

This model is linked to the PilotUser model via a ForeignKey relationship.