Create autonomous drone sentry

## Description
Create a lone drone with its own behaviour tree. It does not move and is spawned alone by the player. It detects any nearby enemy and based on proximity.

## Acceptance Criteria
- Sentry is stationary
- Sentry has its own icon
- Sentry Targets its locked on nearby enemy
- Sentry targets bigger enemies first
- Sentry projectile don't affect player
- Gun turrent always points to target

## Implementation

- [x] Create sentry object
- [x] Add spawning logic
- [ ] Add sentry pickup
- [x] Add rotating sentry turrent
- [x] Create sentry projectile bullet
- [ ] Implement targeting and firing logic
- [ ] Add sentry lifetime to destroy logic

## Notes