Create sentinel portals enemies

## Description
Create sentinel portal enemies. They don;t attack the player but my spawn more enemies. On appearance, other enemies can portal in.

## Acceptance Criteria
- Create sentinel portal sprite
- Create sentinel portal behaviour tree
- Ensure that the sentinel directs the spawners

## Implementation
- [x] Create sentinel portal enemy
- [x] Add behaviour tree for sentinel portal
- [x] Implement sentinel summoning ability

## Notes