Add perplexity as AI Service

### Prerequisites

- [x] The service is not included in the list of [Blocked services](https://github.com/AdguardTeam/HostlistsRegistry/blob/main/assets/services.json).
- [x] This is a popular service and it makes sense to have it blocked;

### Service name

Perplexity

### Explanation

Add one to the popular AI service