Add manual security group handling and expose SG ID

[msk-stackit]

Description

This PR enhances the stackit_loadbalancer resource by introducing advanced networking capabilities for managing security groups, particularly for cross-network and cross-project use cases.

Two new attributes have been added:

disable_security_group_assignment (Optional, Bool): This new root-level flag allows users to disable the automatic creation and assignment of security groups to the load balancer's targets. This is required when targets are located in a different network or project than the load balancer itself, giving users full control over the networking setup.

security_group_id (Computed, String): This new computed attribute exposes the ID of the load balancer's own internal egress security group. Users can reference this ID in their own security group rules (e.g., as the remote_security_group_id) to manually permit traffic from the load balancer to their targets.

Together, these features enable secure and explicit network configurations for complex architectures. This change includes updates to the resource and data source schemas, all necessary business logic, comprehensive unit and acceptance tests, and updated documentation to guide users through the new workflow.

relates to #1150

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[marceljk]

Can you please rebase you branch?
Please take care, that the new import statements in the example are at the end of it